## Posts tagged ‘logic’

### Is Your Gödel Too Tight?

I don’t care what Stevie Nicks says, thunder does not only happen when it’s raining. And sorry, Kelly Clarkson, I’m not standing at your door because I’m sorry.

Logical fallacies are rampant in song lyrics. (Don’t even get me started.) I’m therefore hopeful that you won’t attempt to channel your inner songwriter while trying to solve the following logic puzzles, arranged roughly in order of difficulty.

**Here’s Looking at You**

Jack is looking at Anne, and Anne is looking at George. Jack is married, George is not. Is a married person looking at an unmarried person?

**Beer is Proof that God Loves Us**

Three people walk into a bar, and the bartender asks, “Would all of you like a beer?” The first says, “I don’t know.” The second says, “I don’t know.” The third emphatically replies, “Yes!”

Why was the third one able to respond in the affirmative?

**Five to the Third**

A five-digit number is equal to the sum of its digits raised to the third power. Alphametically,

*CU**BED* = (*C* + *U* + *B* + *E* + *D*)^{3}

What is the five-digit number?

**Martin Gardner’s Children**

I ran into an old friend, and I asked about her family. “How old are your three kids now?”

She said the product of their ages was 36. I replied, “Sorry, I still don’t know how old they are.”

She then said, “Well, the sum of their ages is the same as the house number across the street.”

“I’m sorry,” I said. “I still don’t know how old they are.”

Finally, she told me that the oldest one has red hair, and I finally realized their ages.

How old are my friend’s children?

**If At First You Don’t Succeed…**

If you take a positive integer, multiply its digits to obtain a second number, multiply all of the digits of the second number to obtain a third number, and so on, the *persistence* of a number is the number of steps required to reduce it to a single-digit number by repeating this process. For example, 77 has a persistence of four because it requires four steps to reduce it to a single digit: 77-49-36-18-8. The smallest number of persistence one is 10, the smallest of persistence two is 25, the smallest of persistence three is 39, and the smaller of persistence four is 77.

What is the smallest number of persistence five?

**The Hardest Logic Puzzle Ever**

Three gods A, B, and C are called, in no particular order, True, False, and Random. True always speaks truly, False always speaks falsely, but whether Random speaks truly or falsely is a completely random matter. Your task is to determine the identities of A, B, and C by asking three yes-no questions; each question must be put to exactly one god. The gods understand English, but will answer all questions in their own language, in which the words for *yes* and *no* are *da* and *ja*, in some order. You do not know which word means which.

(This puzzle is attributed to Raymond Smullyan, but the twist of not knowing which word means which was apparently added by computer scientist John McCarthy.)

### Insanity, the Logic of a Mind Overtasked

I asked my friend what he knew.

I don’t know anything.

Who are you, the Barber of Seville? You know at least one thing, namely that you don’t know anything. A contradiction!

So he corrected himself.

I don’t know nothing.

Ha! If you don’t (-) know nothing (-), then you must know something (+). A double negative.

It was at that point that my friend stopped being my friend.

This is what logic will do to your social life.

Logic: a systematic method for getting the wrong conclusion, with confidence.

But it can also be useful for solving problems.

John had 50 candy bars, and he ate 45 of them. Now what does he have?

Diabetes!

And we end this silliness with three pieces of advice from the king of bad logic, Yogi Berra.

- Never answer an anonymous letter.
- Nobody goes there anymore. It’s too crowded.
- You better cut the pizza in four pieces. I’m not hungry enough to eat six.

Hope you enjoyed or did not enjoy this post (but not both).

### Can’t Argue with That

My momma always told me:

Don’t break a person’s heart; they only have one. Break their bones; they have 206.

Who can argue with that logic? Here are some other logical statements with which you won’t want to argue, either.

I asked my wife what she wanted for her birthday. She said, “Nothing would make me happier than diamond earrings.” So, I got her nothing.

I find it strange that my advisor always begins conversations with me by saying, “You haven’t heard a word I’ve said, have you?”

It doesn’t matter if the glass is half empty or half full; either way, there is room for more alcohol.

I only drink twice a year: when it’s my birthday, and when it’s not.

My math teacher just fell in a wishing well. Go figure! I never knew they worked.

My advisor says I’ll never graduate because I’m lazy. But I just can’t take that kind of criticism. I was going to kill myself… but the gun’s, like, way over there.

Don’t judge a book by its cover… my math book has a picture of someone enjoying himself.

A grad student told his friend, “My girlfriend hates it when I sneak up behind her and kiss her on the cheek. But according to her lawyer, she also hates it when I call her my girlfriend.”

I got a tattoo of Chinese symbols on my arm that reads, “I don’t know. I don’t speak Chinese.” So when someone asks what it says…

Boy: I hate my math professor. He’s a terrible lecturer, he has bad breath, and he laughs at his own jokes.

Girl: Who’s your professor?

Boy: Dr. Jacoby.

Girl: Do you know who I am?

Boy: No.

Girl: I’m Dr. Jacoby’s daughter.

Boy: Do you know who I am?

Girl: No.

Boy: Good.

### Silly-Gisms

A *syllogism* is a logical argument in which the conclusion is inferred from two premises. As an example:

All men are animals.

All animals are mortal.

Therefore, all men are mortal.

My favorite syllogism comes from comedian Richard Geni who delivered the following in one of his stand-up routines:

Love is blind.

God is love.

Therefore, Ray Charles is God.

Here are a few other syllogisms that are a little more mathematical, though equally silly.

Ten percent of all car thieves are left-handed.

All polar bears are left-handed.

If your car is stolen, there’s a 10% chance it was taken by a polar bear.Thirty-nine percent of unemployed men wear glasses.

Eighty percent of employed men wear spectacles.

Therefore, work causes bad vision.Every second, 4,000 cans are opened around the world.

Every second, ten babies are conceived around the world.

Therefore, each time you open a can, you have a 1 in 400 chance of becoming pregnant. (Be careful!)

### There Must Be Some Misunderstanding

The WordPress blogging system comes with administrative controls, and it allows me to see what folks are searching for when they reach my blog. One of the search phrases that showed in the admin area today:

“math jokes – if you get them, you don’t have friends”

That’s not true. And I can prove it’s not true, by showing that its contrapositive is untrue.

P = You get math jokes.

Q = You have friends.

Then this argument is

If P, then -Q.

The contrapositive is, “If you have friends, then you don’t get math jokes.” Symbolically,

If Q, then -P.

I have friends. Or, I have at least one friend, which is all I need to prove the truth of Q. And given that I’m the author of this blog, then clearly I get math jokes. Consequently, the contrapositive is untrue; and by the Law of Contrapositives, then the original statement is untrue. Q.E.D.

Sticking to formal logic, Brent Yorgey recently reviewed Math Jokes 4 Mathy Folks on his blog The Math Less Traveled. In his review, he stated that this was one of his favorite jokes in the book:

A logician said to his son, “If you don’t eat your vegetables, you can’t have any ice cream.” Upon hearing this, the son choked down a plate of broccoli, and his father, duly impressed, sent him to bed without any ice cream.

Upon reading this review, my publisher said, “What a terrible joke he chose to highlight! I don’t understand why it’s funny. The logician just sounds cruel!”

I then had to explain that the humor derives from the logical error known as denying the antecedent. The logician said, “If you don’t eat your vegetables, then you can’t have any ice cream.” It is a common mistake for folks to assume that the logician’s statement is equivalent to, “If you eat your vegetables, then you can have ice cream.” But it’s not. The second statement is the inverse of the original statement, and a statement and its inverse are not logically equivalent. The logician asserted that if the son didn’t eat the vegetables, then he would not get ice cream; however, he did not guarantee that his son would get ice cream for eating his vegetables.

Yeah, you’re right. Even if you understand it, it’s still not very funny.

### Three Jokes from Italy

Thanks to Maurizio Codogno, who bought my book from bookdepository.co.uk (they were kind enough to ship it to him in Italy), and who also shared a few jokes that weren’t in *Math Jokes 4 Mathy Folks*. Enjoy them, hot off the presses from Milan!

**Question and Answer**

Q: What is the difference between a mathematician and a physicist?

A: The mathematician thinks there is only one straight line that passes through two points; the physicist, however, needs more data.

**Logic Exam**

A student asks his logic professor, “Sir, did I pass or fail the exam?”

The professor replies, “Yes.”

**Quantum Mechanics**

Every Friday night, a mathematician goes to the pub, sits on the next-to-last stool, turns to the last stool, and asks to a non-existent woman if she would like a drink. The mathematician returns every Friday night for a year, yet the bartender says nothing.

Finally, the last Friday before summer break, the bartender asks the mathematician, “Excuse me, sir. You are clearly aware that there is no woman sitting in that chair. Why do you keep talking to an empty stool?”

The mathematician responds, “According to quantum mechanics, an empty space is not really void. Virtual particles materialize and disappear at every instant. Nobody knows whether the appropriate wave function collapses in such a way that a beautiful girl will appear out of nowhere.”

The bartender raises his eyebrow. “Really? That’s interesting. But couldn’t you just ask one of the women already in the bar if she’d like a drink? Who knows, maybe one of them would say yes.”

The mathematican laughs. “Oh, sure!” he says. “And what is the probability of

thathappening?”