## Posts filed under ‘Uncategorized’

### Number Challenge from Will Shortz and NPR

Typically, the NPR Sunday Puzzle involves a word-based challenge, but this week’s challenge was a number puzzle.

[This challenge] comes from Zack Guido, who’s the author of the book

Of Course! The Greatest Collection of Riddles & Brain Teasers for Expanding Your Mind. Write down the equation

65 – 43 = 21You’ll notice that this is not correct. 65 minus 43 equals 22, not 21. The object is to

move exactly two of the digits to create a correct equation. There is no trick in the puzzle’s wording. In the answer, the minus and equal signs do not move.

Seemed like an appropriate one to share with the MJ4MF audience. Enjoy!

### The Homework Inequality: 1 Great Problem > 50 Repetitive Exercises

Yesterday, my sons Alex and Eli were completing their homework on fraction operations, which included 39 problems in 4 sets, wherein each problem in a set was indistinguishable from its neighbors.

The worksheet contained 12 problems of fraction addition, 9 problems of fraction subtraction, 9 problems of fraction multiplication, and 9 problems of fraction-times-whole-number multiplication. That’s 39 problems of drudgery, when 10 problems would’ve been sufficient. Here’s a link to the worksheet they were given, if you’d like to torture your children or students in a similar way:

Frustrated by the monotony of the assignment, I told the boys they didn’t have to do all of the problems, and they could stop when they felt that they had done enough from each set.

“No,” said Alex. “We’re supposed to do them all.”

My sons are responsible students, but I’m frustrated by teachers who take advantage of their work ethic. Just because they’re *willing* to complete 50 exercises for homework doesn’t mean they *should be assigned* 50 exercises for homework.

My colleague at Discovery Education, Matt Cwalina, puts it this way:

Some say that a picture is worth a thousand words. I say,

A great problem is worth a thousand exercises.

Personally, I would much rather have students think deeply about one challenging problem than mindlessly complete an entire worksheet. Luckily, my sons take after their daddy and love number puzzles, so I spontaneously created one.

Find three fractions, each with a single-digit numerator and denominator, that multiply to get as close to 1 as possible. Don’t repeat digits.

Eli started randomly suggesting products. “What about 4/5 × 6/7 × 9/8?” He’d work out the result, say, “I think I can do better,” then try another. And another. And another. Finally, he found a product that equaled 1. (No spoiler here. Find it yourself.)

Alex eventually found an answer, too. At the bottom of his homework assignment, he added a section that he titled “Bonus” where he captured his attempts:

I don’t know exactly how many calculations Eli completed while working on this problem, but I know that Alex completed at least seven, thanks to his documentation. Wouldn’t you agree that completing several fraction computations while thinking about this more interesting problem is superior to doing a collection of random fraction computations with no purpose?

There is a preponderance of evidence (see Rohrer, Dedrick, and Stershic 2015; definitely check out **Figure 4** at the top of page 905) that **massed practice** — that is, completing a large number of repetitions of the same activity over and over — is counterproductive. Unfortunately, massed practice feels good because it results in short-term memory gains, which trigger a perceived level of mastery; but, it doesn’t lead to long-term retention. Moreover, students who learn a skill by practicing it repeatedly get really good at performing that skill *when they know it’s coming*; but, two months down the road, when they need to use that skill in an unfamiliar context because it’s not on a worksheet titled “Lesson 0.1: Adding and Multiplying Fractions,” they’re less likely to remember than if they had used more effective practice methods. One of those more effective methods is **interleaving**, which involves spacing out practice over multiple sessions and varying the difficulty of the tasks. Whether you’re trying to learn how to integrate by parts or how to hit a curve ball, be sure to make your practice exercises a little more difficult than you’re used to. Know that interleaving your practice will not feel as good as massed practice while you’re doing it; but later, you’ll feel better due to improved memory, long-term learning, and mastery of skills.

Interleaving is one of the reasons I love the ** MathCounts School Handbook**, which can be downloaded for free from the MathCounts website. The topics covered by the 250 problems in the

*School Handbook*run the gamut from algebra, number sense, and probability, to geometry, statistics, combinatorics, sequences, and proportional reasoning — and any given page may contain problems of any type! Veteran MathCounts coach Nick Diaz refers to this mixture as “shotgun style,” meaning that students never know what’s coming next. Consequently, similar problems are not presented all at once; instead, students are exposed to them several days or perhaps weeks apart. Having to recall a skill that hasn’t been used for a while requires more effort than remembering what you did just five minutes ago, but the result is long-term retention. It’s doubtful that the writers of the first

*MathCounts School Handbook*knew the research about interleaving and massed practice… but they clearly knew about effective learning.

The other reason I like the *MathCounts School Handbook* is the difficulty level of the problems. Sure, some of the items look like traditional textbook exercises, but you’ll also find a lot of atypical problems, like this one from the *2017-18 School Handbook*:

If

p,q, andrare prime numbers such thatpq+r= 73, what is the least possible value ofp+q+r?

That problem, as well as the fraction problem that I created for Alex and Eli, would both fall into the category of open-middle problems, which means…

- the beginning is closed: every students starts with the same initial problem.
- the end is closed: there is a small, finite number of unique answers (often, just one).
- the middle is open: there are multiple ways to approach and ultimately solve the problem.

Open-middle problems often allow for implicit procedural practice while asking students to focus on a more challenging problem. This results in a higher level of engagement for students. Moreover, it reduces the need for massed practice, because students are performing calculations while doing something else. You can find a large collection of open-middle problems at **www.openmiddle.com**, and the following is one of my favorites:

Use the digits 1 to 9, at most one time each, to fill in the boxes to make a result that has the greatest value possible.

It’s a great problem, because random guessing will lead students to combinations that work, but it may not be obvious how to determine the greatest possible value. Consequently, there’s an entry point for all students, the problem offers implicit procedural practice, and the challenge of finding the greatest value provides motivation for students to continue.

I have a dream that one day, in traditional classrooms where 50 problems are assigned for homework every night, where procedural fluency is valued over conceptual understanding; that one day, right there in those classrooms, students will no longer think that math is simply a series of disparate rules with no purpose, but instead will experience the joy of attempting and solving challenging problems that inspire purposeful play and, as a side benefit, encourage students to practice the skills they will need to be successful learners.

There are myriad resources available so that teachers and parents can encourage their students to engage in these kinds of problem-solving activities, so it is my hope that this dream is not too far away.

As a special bonus for reading to the end, check out this ** Interleaved Mathematics Practice Guide** that Professor Doug Rohrer was kind enough to share with me (and now, with you).

### Hiking Routes, Square Roots, and Trail Ratings

* Not all those who wander are lost.*

You may have read that line in Gandalf’s letter to Frodo Baggins in *The Fellowship of the Ring*, but nowadays you’re more likely to see it on the t-shirts and bumper stickers of hikers.

Hiking is a popular sport, and the 47 million Americans who reported that they’ve taken a hike in the past 12 months (Statista) had a lot of different trails to choose from: American Trails maintains a database of over 1,100 trails, and Backpacker‘s list of America’s Best Long Trails offers an impressive 39,000 combined miles. Plus, there are thousands of miles of trail not on either of those lists. With so many options, how are you supposed to choose?

A wealth of information is provided for most hiking trails. But while some information — like distance and elevation gain — is absolute, other information leaves room for interpretation. What does it mean when the Craggy Pinnacle Trail just outside Asheville, NC, is described as a “moderate” hike? The Explore Asheville website says,

Moderate hikes could range anywhere from a few to ten miles with an elevation gain up to 2,000 feet.

By those standards, a three-mile hike with a 10% grade would be considered moderate. No, thank you.

Unfortunately, there is no standardized system for determining trail difficulty. Most of the time, the trail rating is a nebulous **qualitative** combination based on an examination of the terrain, trail conditions, length, elevation gain, and the rater’s disposition.

But I tip my hat to the good folks in Shenandoah National Park who have attempted to **quantify** this process. Their solution? The simple formula

where *g* is the elevation gain (in feet) and *d* is the distance (in miles). The value of *r* then corresponds to a trail rating from the following table:

Numerical Rating |
Level of Difficulty |
Estimated Average Pace (miles per hour) |

< 50 | Easiest | 1.5 |

50-100 | Moderate | 1.4 |

100-150 | Moderately Strenuous | 1.3 |

150-200 | Strenuous | 1.2 |

> 200 | Very Strenuous | 1.2 |

Elevation gain is defined as the cumulative elevation gain over the entire hike. So if the hike climbs 300 feet over the first mile, then descends 500 feet over the next 2 miles, then goes back up 200 feet to return to the start, the elevation gain is reported as 300 + 200 = 500 feet.

Old Rag is one of the most popular hikes in northern Virginia. Known for the half-mile rock scramble near the top, this trail boasts an impressive 2,415 feet of elevation gain over 9.1 miles. Applying the formula,

which means Old Rag’s level of difficulty would be “very strenuous.”

This formula could lead to several activities for a middle or high school classroom:

- Draw an elevation map depicting a trail on which any type of hike (from
*easiest*to*very strenuous*) would be possible, depending on how far a person hiked. - With distance on the horizontal axis and elevation gain on the vertical axis, create a graph that shows the functions for
*easiest*to*very strenuous*hikes. (See Figure 1.) - If you were on a trail with an average elevation gain of 300 feet per mile, how long would you have to hike for it to be considered a
*moderately strenuous*hike? - If one 5-mile hike is rated “easiest” and another 5-mile hike is rated “strenuous,” what’s the minimum possible difference in elevation gains for the two trails?

Students could also do a comparison between this trail rating formula and the geometric mean, if you wanted to go really crazy.

Feel free to drop some of your ideas into the comments.

Just as every good hike comes to an end, so must this blog post. But not before we laugh a little.

As it turns out, there’s a math joke about hiking…

An actuary has been walking for several hours when the trail ends at the edge of a river. Having no idea how to cross, she sees another hiker on the opposite bank, and she yells, “Hey, how do I get to the other side?”

The man across the river — a math professor — looks upstream, then downstream, then thinks a bit and finally says, “But you

areon the other side!”

It’s a math joke about hiking as much as any joke about any topic is a math joke, if you insert the correct professions.

There’s a great non-math joke about hiking, too…

A fish is hiking through a reservoir when he walks into a wall. “Dam!” he says.

And there is a very mathematical list about hiking, which might be considered a joke if so many of the observations weren’t true…

**Eight Mathematical Lessons from the Trail**

- A pebble in a hiking boot will migrate to the point of maximum irritation.
- The distance to the trailhead where you parked remains constant as twilight approaches.
- The sun sets at two-and-a-half times its normal rate when you’ re trying to reach the trailhead before dark.
- The mosquito population at any given location is inversely proportional to the effectiveness of your repellent.
- Waterproof rainwear isn’t. But, it is 100% effective at containing sweat.
- The width of backpack straps decreases with the distance hiked. To compensate, the weight of the backpack increases.
- The ambient temperature increases proportionally to the amount of extra clothing in your backpack.
- The weight in a backpack can never remain uniformly distributed.

**Go take a hike!**

### My Insecurity Over Security Codes

Every time I attempt to access one of my company’s applications via our single sign-on (SSO) system, I’m required to request a validation code that is then sent to my smartphone, and then I enter that code on the login page.

It’s a minor nuisance that drives me insane.

The purpose of the codes are to provide an additional level of security, but given how un-random the codes seem to be, it doesn’t feel very secure to me. This screenshot shows some of the codes that I’ve received recently:

Here’s what I’ve observed:

- Every security code contains 6 digits.
- The first 3 digits in the code form either an arithmetic or geometric sequence, or the first 3 digits contain a repeated digit.
- Similarly, the last 3 digits in the code form either an arithmetic or geometric sequence, or the last 3 digits contain a repeated digit.

As an example, one of the codes in the screenshot above is 421774. The first 3 digits form the (descending) geometric sequence 4, 2, 1, and the digit 7 appears twice in the second half of the code.

I believe the reason for these patterns is to make the codes more memorable to those of us who have to transcribe them from our phones to our laptops.

This got me thinking. The likelihood of someone correctly guessing a six-digit code is 1 in 1,000,000. But what is the likelihood that someone could correctly guess a six-digit code if it adheres to the rules above?

If you’d like to answer this question on your own, stop reading here. To put some space between you and my solution, here’s a security-related joke:

“I don’t understand how someone stole my identity,” Lily said. “My PIN is so secure!”

“What’s your PIN?” Millie asked.

“The year of Knut Långe’s death,” Lily replied.

“Who is Knut Långe?”

“A King of Sweden who usurped the throne from Erik Eriksson.”

“And what year did he die?”

“1234.”

(Incidentally, Data Genetics reviewed 3.4 million stolen website passwords, and they found that 1234 was the most popular four-digit code. The researchers claimed that they could use this information to make predictions about ATM PINs, too, but I don’t think so. All this shows is that 1234 is the most commonly *stolen* password, and therefore this inference suffers from survivorship bias. Without having data on all the codes that were *not* stolen, it’s impossible to make a reasonable claim. But, I digress.)

To determine the number of validation codes that adhere to the patterns I observed, I started by counting the number of arithmetic sequences. With only 3 digits, there are 20 possible sequences:

- 012
- 024
- 036
- 048
- 123
- 135
- 147
- 159
- 234
- 246
- 258
- 345
- 357
- 369
- 456
- 468
- 567
- 579
- 678
- 789

But each of those could also appear in reverse (210, 975, etc.), giving a total of 40.

There are far fewer geometric sequences; in fact, only 3 of them:

- 124
- 139
- 248

And again, each of those could appear in reverse, giving a total of 6.

Finally, there are 10 × 9 × 8 = 720 three-digit numbers with no repeated digits, which means there are 1,000 ‑ 720 = 280 numbers with a repeated digit. (Here, “number” refers to any string of 3 digits, including those that start with a 0, like 007 or 092.)

Consequently, there are 40 + 6 + 280 = 326 possible combinations for the first 3 digits and also 326 combinations for the last 3 digits, which gives a total of **326 × 326 = 106,276 possible validation codes**.

That means that it would be about 10× more likely for a phisher to correctly guess a validation code that follows these rules than to guess a completely random six-digit code. But said another way, the odds are still significantly against a phisher who’s trying to steal my code. And quite frankly, if someone wants to exert that kind of effort to pirate my access to Microsoft Word online, well, I say, go for it.

### 8-15-17

Today is a glorious day!

The date is 8/15/17, which is mathematically significant because those three numbers represent a Pythagorean triple:

But August 15 has also been historically important:

- It’s the birthday of some famous people, including Jennifer Lawrence, Kerri Walsh-Jennings, Napoleon Bonaparte, Julia Child, and Ben Affleck, as well as some not-so-famous people, including one of my sisters, one of my aunts, one of my uncles, and my maternal grandfather.
- The Mayflower departed from Southampton, England, on August 15, 1620.
- The Panama Canal opened to traffic on August 15, 1914.
*The Wizard of Oz*premiered at Grauman’s Chinese Theater on August 15, 1939, and exactly 40 years later,*Apocalypse Now*was released.- In 1945, the Japanese surrendered on August 15.

But as of today, August 15 has one more reason to brag: It’s the official publication date of a bestseller-to-be…

Like its predecessor, this second volume of math humor contains over 400 jokes. Faithful readers of this blog may have seen a few of them before, but most are new. And if you own a copy of the original * Math Jokes 4 Mathy Folks*, well, fear not — you won’t see any repeats.

What kind of amazing material will you find on the pages of * More Jokes 4 Mathy Folks*? There are jokes about school…

An excited son says, “I got 100% in math class today!”

“That’s great!” his mom replies. “On what?”

The son says, “50% on my homework, and 50% on my quiz!”

There are jokes about mathematical professions…

An actuary, an underwriter, and an insurance salesperson are riding in a car. The salesperson has his foot on the gas, the underwriter has her foot on the brake, and the actuary is looking out the back window telling them where to go.

There are Tom Swifties…

“13/6 is a fraction,” said Tom improperly.

And, of course, there are pure math jokes to amuse your inner geek…

You know you’re a mathematician if you’ve ever wondered how Euler pronounced Euclid.

Hungry for more? Sorry, you’ll have to buy a copy to sate that craving.

To purchase a copy for yourself or for the math geeks in your life, visit **Amazon**, where * MoreJ4MF* is already getting rave reviews:

For quantity discounts, visit **Robert D. Reed Publishers**.

### Mo’ Math Limericks

I’ve posted limericks to this blog before. Quite a few, in fact.

But a friend recently sent me *The Mathematical Magpie*, a collection of math essays, stories and poems assembled by Clifton Fadiman and published by Simon and Schuster in 1962. Coincidentally, one section of the book is titled *Comic Sections*, the name of a mathematical joke book written by Des MacHale in 1993. (I contacted Professor MacHale several years ago, and he suggested that we swap books. Best. Trade. Ever.) Des MacHale is Emeritus Professor at the University of Cork, a mere 102 km from Limerick, Ireland… which brings us full circle to today’s topic.

*The Mathematical Magpie* contains quite a few limericks, one of which you have likely heard before:

There was a young lady named Bright,

Who traveled much faster than light.

She started one day

In the relative way,

And returned on the previous night.

Despite a variety of other claims, that limerick was written by Professor A. H. Reginald Buller, F.R.S., a biologist who received £2 when the poem was published in *Punch*, and he “was more excited at the check than he was later when his book on fungi was published.”

You may not, however, be familiar with Professor Buller’s follow-up limerick about Miss Bright:

To her friends said the Bright one in chatter,

“I have learned something new about matter:

As my speed was so great

Much increased was my weight,

Yet I failed to become any fatter!”

Here are a few other limericks that appear in *The Mathematical Magpie*:

There was an old man who said, “Do

Tell me how I’m to add two and two?

I’m not very sure

That it doesn’t make four —

But I fear that is almost too few.

Anon.The topologist’s mind came unguided

When his theories, some colleagues derided.

Out of Möbius strips

Paper dolls he now snips,

Non-Euclidean, closed, and one-sided.

Hilbert Schenck, Jr.A mathematician named Ray

Says extraction of cubes is child’s play.

You don’t need equations

Or long calculations

Just hot water to run on the tray.

L. A. GrahamFlappity, floppity, flip!

The mouse on the Möbius strip.

The strip revolved,

The mouse dissolved

In a chronodimensional skip.

Frederick Winsor

And though it’s not a limerick, this one is just too good not to include for your enjoyment:

A diller, a dollar,

A witless trig scholar

On a ladder against a wall.

If length over height

Gives an angle too slight,

The cosecant may prove his downfall.

L. A. Graham

Finally, I leave you with a MJ4MF original:

With my head in an oven

And my feet on some ice,

I’d say that, on average,

I feel rather nice!

**Got any math poems or limericks you’d like to share? We’d love to hear them!**