Wednesday, October 21, 2009

10,000 Left Shoes

Who would ever want to have 10,000 left shoes?

Well, here is a story that has stuck in my head for many years. I may be off on the specifics but I believe the just of it is true. And, if not, it could have happened or, in storytelling parlance, it’s plausible.

As I recall, in the mid 1960’s in Israel I read in the paper that a shipment of 10,000 left shoes had arrived in the port of Haifa from Italy. At the time, custom duty rates in Israel were extremely high, especially on such luxury imports. So the importer did not claim the shoes. Besides, what would he do with 10,000 left shoes? Stuck with this load, the custom office did what most custom agencies do with abandoned stuff, it auctioned it off. But no one had any use for 10,000 left shoes so no one made any bid. Well, eventually the original importer made a very low bid and the custom office was happy to get rid of the shoes that took space in its warehouse. So the importer got his 10,000 left shoes practically paying no duty.

A month or two later a shipment of 10,000 right shoes arrived in the port of Haifa and the same importer did not claim them because he did not want to pay the duty…

Surely, you have figured out by now the rest of the story.

Where is the math in this story?

It has to do with sets and how sets can be split, added up and rearranged. A pair of shoes is a set, a very useful set. It can be sold. 10,000 pairs is a set of 10,000 such useful sets. Splitting these pairs into 10,000 left shoes and 10,000 right shoes generates 2 sets containing 10,000 of useless shoes, which no one wants to buy. But then, whoever has both sets can recombine them to form the original 10,000 sets of sellable pairs of shoes.

Friday, June 12, 2009

Right on Time

A student returned home from a date at 3 AM. Her parents were very upset, "You're late! You said you'd be home by 11:45!"

"Actually," the girl replied, "I'm right on time. I said I'd be home by 1/4 of 12."

[Origin: unknown; several variations of this joke appear on various web; a version of this joke, submitted by Zhang Wenyi, was published in Reader's Digest, July 2009, "Laugh!:)", p. 27]

Sunday, February 22, 2009

Mathematical Chuck Norris Facts

For the unfamiliar, there is a class of jokes about how awesome Chuck Norris is. Here I will post those with mathematical twist.

• Chuck Norris counted to infinity, twice.
[, as of 2009-02-22]

• Chuck Norris knows the last digit of pi.
[, as of 2009-02-22]

• Chuck Norris can divide by zero.
[, as of 2009-02-22]

• If you have five dollars and Chuck Norris has five dollars, Chuck Norris has more money than you.
[, as of 2009-02-22]

• The square-root of -1 is not imaginary. It's just hiding from Chuck Norris.
[Ben, 2009-02-22]

• The shortest distance between two points is Chuck Norris.

• The square root of 2 is rational number for Chuck Norris.

• Chuck Norris can square the circle, double the cube and trisect an angle using only his fingers for a compass and his arm for a straight edge.

Saturday, February 21, 2009

Subtraction Without Borrowing

clipped from

Subtraction Without Borrowing by MathVentures [Video Prototype 01]

This is my first video prototype. Lots of room for improvement,

Clay Shirky: Institutions vs. collaboration

clipped from

Clay Shirky: Institutions vs. collaboration

Monday, February 9, 2009

Testing Probability

Flummoxed by his true-false final exam, a student decides to toss a coin up in the air. Heads means true; tails, false. Thirty minutes later, he is done, well before the rest of the class. But then the student startsd flipping the coin again. And soon he's swearing and sweating over each question.

"What's wrong?" asks the concern teacher.

"I'm rechecking my answers," says the student.

[Comic Wendell Potter, Laugh!:), Reader Digest, March 2009, p. 81]

Uri's Comment: It is interesting to note that the student can change any answer that is not confirmed without affecting the probable grade of the test. Of course, for this to be true, the number of questions should be as large as possible. Considering that (a) it took the students 30 min. to finish the test and (b) it takes under 6 seconds to toss a coin and jot down the result, the test could have consisted of 150-300 questions (no need to spend time on reading each question). This test consists of a sufficient number of questions for probability to determine the overall grade.