## Posts tagged ‘probability’

### Sock Probability

I don’t know if problems like the following are famous, but there sure are a lot of them online — Cut the Knot, Stack Exchange, and Braingle, for example — and they’re typical for a high school classroom or middle school math competition:

There are 14 red, 6 orange, 10 yellow, 8 green, 4 blue, 12 indigo, and 2 violet socks in my sock drawer. How many socks must I randomly remove from the drawer to guarantee that I have two socks of the same color?

You may or may not know the answer, but the problem itself leads to a follow-up question:

Why the hell do I own socks in every color of the rainbow?

That’s just weird. But if you can get past that, here is a related problem:

If I randomly remove two socks from the drawer, what is the probability that they form a matching pair?

As it turns out, I’m something of a sock aficionado. (Yeah, it’s weird, but surely not surprising. I mean, I write a math jokes blog. You didn’t think I was normal, did you?) Although the context above is fictitious, I am indeed the owner of three pairs of identical socks that look like those shown in the picture. And yes, those are *my* feet and ankles. My mathematical sexy runs all the way down to my toes.

Here’s a close-up of one of them, in case you can’t see it in the larger picture:

That’s a letter **R**, because these socks are specially designed for each foot. The other sock has a letter **L**. (Duh.)

This leads to another mathematical question, more real-world than those above:

I just finished washing these three pairs of socks. While folding them, I selected two socks at random and rolled them together. What’s the probability that there’s one R and one L?

The answer, of course, is **zero**.

Yes, I know that *theoretically* the answer should be **3/5**. But theory doesn’t match practice in this case. When I do my laundry, I sometimes forget to pay attention to the R and the L, and my sock drawer invariably results in one pair of two R’s, one pair of two L’s, and one correctly matched pair. And then when I wake up at 5:30 a.m. and put on my socks in the dark (so as not to rouse my wife from slumber), my feet feel all weird. The one with the wrong sock starts tingling, so I have to remove the socks and choose another pair entirely.

Similarly, here’s another real-world problem, based on my sock experience:

If there are 10 socks in a load of laundry that I place in the washer and then transfer to the dryer, how many socks will remain when the load is finished drying?

**Nine.** Yes, I *know* it’s a cliche. Everyone makes jokes about losing socks. It’s so overdone that the National Comedian’s Guild has declared a moratorium against them. But, *I’m not joking*. I can’t remember the last time I did a load of laundry and wasn’t missing a sock. I now have a drawer filled with unmatched socks, each like Tiger Woods longing for the return of its Lindsey Vonn.

Sadly, this post is going public just a little too late. **Lost Socks Memorial Day** was **May 9**, so we just missed that one. Likewise, we missed **No Socks Day** on **May 8**. But there are other holidays in the coming months when you can celebrate the amazing undergarments that protect our feet from our shoes:

- July (exact date TBD):
**Red Socks Day**(commemorating Sir Peter Blake) - October 4:
**Odd Socks Day**(Australia) - January (every Friday):
**Snow Sock Day**

And though not an official holiday, there are unlimited **Crazy Sock Days** happening at elementary, middle, and high schools near you.

99% of socks are single, and you don’t see them crying about it.

How do engineers make a bold fashion statement?

They wear their dark grey socks instead of the light grey ones.Somewhere, all of my socks, Tupperware lids, and ball point pens are hanging out together, just laughing at me.

—

Because I know you won’t be able to sleep tonight…

- I need to remove
**8 socks**from my sock drawer to guarantee a color match. - I don’t actually own socks in
**every color**of the rainbow. Just most colors. - The probability of selecting two socks from my drawer and getting a matching pair is
**23/140**.

### Shoestring Probability

March 15 is Shoe the World Day. And April 5 is One Day Without Shoes Day.

Shoes and math have a lot in common.

A shoe salesman consults a mathematician on what size shoes to keep in stock. The mathematician tells him, “There is a simple equation for that,” and shows him the Gaussian normal distribution.

The shoe salesman stares at the equation for a while, then asks, “What’s that symbol?”

“That’s the Greek letter π.”

“What is π?”

“The ratio between the circumference and the diameter of a circle.”

The shoe salesperson thinks for a minute. “What the hell does a circle have to do with shoes?”

As it turns out, there are at least 43 different ways to arrange the laces on your shoes. My favorite is the hexagram method:

But there are some fun things to do with your shoelaces other than lacing up your kicks. Here’s one.

Take the shoelaces out of your shoes. Fold the shoelaces in half and hold them in one hand so that the four aglets are exposed but the rest of the shoelaces are hidden in your palm. Like this:

Have a friend select two of the aglets and tie those ends together (I recommend a square knot). Then, have your friend tie the other two ends together. Finally, offer your friend the following wager:

You give me $1 if you formed one large loop.

I’ll give you $1 if you didn’t.

Is it a fair bet?

Too easy? Then try this. Take your shoelaces *and* your friend’s shoelaces, fold them in half, and then expose the eight aglets. Choose two at a time and tie them together. The wager remains the same.

Now is it fair?

Is it possible to create a fair wager with any number of shoelaces? If so, how many?

### When **Super Bowl XLIX** Starts to Bore You…

I appreciate that the Super Bowl unites all Americans in their inability to read Roman numerals.

(Caution — oxymoron ahead!) If you’re a smart football fan, then you’ve invited folks to your house to watch the game, so you don’t have to drive home drunk on a cold Sunday night in February.

If this year’s Super Bowl is like last year’s, your guests will be bored by halftime, with the Seahawks leading 22‑0.

Or if deflated balls are used, the Patriots could be leading 38‑7 at the end of the third quarter, like when they played the Colts two weeks ago.

In either case, you’ll need something to keep your guests entertained between commercials and after the Katy Perry halftime show. I suggest the following problem.

Imagine that the NFL has eliminated divisions, and there are just two conferences with 16 teams each. To simplify things, every team plays the other 15 teams in their conference exactly once each. At the end of the regular season, what is probability that every team within a conference has a different record? (Assuming, of course, that each team is equally likely to win on any given Sunday, and assuming that ties are not possible.)

It’s not a trivial problem, so I’ll give you some time to think about it. The answer will be revealed on Monday, February 2, 2015; that is, **after** the game.

### What’s Your Problem?

Problems in the MathCounts School Handbook are presented “shotgun style,” that is, a geometry problem precedes a logic puzzle and follows a probability question. (I worked for MathCounts for seven years and then served as a writer and chair of their Question Writing Committee, so I’m not unbiased.)

By comparison, textbooks often present 50 exercises on the same topic, each one only minimally different from the previous one. That tips the hand to students, methinks, and makes them realize, “Oh, I just need to do the same thing.” I prefer the MathCounts approach, where students have to dig into their bag of tricks to find a viable solution strategy.

With that in mind, here are a few problems I’ve encountered recently, each one not like the others.

**Problem 1.** The simple polygon is made from 73 squares, connected at their sides. What is the perimeter of the figure?

**Problem 2.** What is the expected number of times that a six-sided die must be rolled to get each number 1–6?

**Problem 3.** A wall is to be constructed from 2 x 1 bricks (that is, bricks that are twice as long in one direction as the other). A strong wall must have no **fault lines**; that is, it should have no horizontal or vertical lines that cut entirely through a configuration, dividing it into two pieces. What is the minimum size of a wall with no fault lines? The figure below shows a 3 × 4 wall that has both horizontal and vertical fault lines.

**Please share great problems you’ve recently encountered in the Comments.**

No answers, but here are some hints.

*Problem 1. *Look for a pattern.

*Problem 2.* Check out this simulation for the Cereal Box problem.

*Problem 3.* The smallest arrangement without a fault line is larger than 3 × 4 and smaller than 10 × 10.

### Go to Vegas, Saul

Saul is a statistician. He leads a comfortable life — he has tenure at a respected university, an impressive list of publications to his credit, and the admiration of his colleagues. Less than a year from retirement, he hears a voice from above. “Saul, quit your job,” the voice says.

He ignores it.

The next day, the voice returns. “Saul, quit your job.” And the next day. And the day after that. And it becomes more frequent, occupying most of his waking hours as well as his dreams. “Saul, quit your job.”

It continues relentlessly for months. “Enough already!” Saul shouts when he can take no more. He delivers a letter of resignation to his dean that morning.

“Saul, take your life savings out of the bank.”

*I’m not taking out my money*, Saul thinks. But the voice continues relentlessly. “Saul, take your life savings out of the bank.”

After several sleepless nights, he finally gives in. “Now what?” he asks.

“Saul, go to Vegas.”

He buys a ticket to Vegas. When he arrives, the voice tells him, “Saul, go to the blackjack table.”

He obeys.

“Saul, bet all of your money on one hand.”

“That’s insane!” he shouts.

“Saul, bet all of your money on one hand.”

He knows that the voice will continue if he doesn’t listen, so he does it.

He’s dealt an 8 and a king. 18. The dealer is showing a 6.

“Saul, take a card.”

“But the dealer has…”

“Saul, take a card.”

“But the laws of probability…”

“Saul, take a card!”

He takes a card reluctantly. It’s an ace. 19. He sighs relief.

“Saul, take another card.”

“C’mon!”

“Saul, take another card!”

He takes another card. Another ace. 20.

“Saul, take another card.”

“But I have 20!” he shouts.

“Saul, take another card!”

He shakes his head. “Hit me,” he says sheepishly. A third ace. 21.

And the voice booms, “Un-fucking-believable!”

### The Twelve Days of Crisp Math – Day 8

For the **Eighth Day of Crisp Math**, here’s a problem for you. Best of luck solving it before Day 9…

If you choose an answer to this question at random, what is the probability that you will be correct?

A. 25%

B. 50%

C. 60%

D. 25%