Rank Math Trick

December 3, 2010 at 7:04 am 1 comment

The Amazon sales rank for Math Jokes 4 Mathy Folks fluctuates between 20,000 and 300,000. The highest rank to date was 11,394, which occurred July 7, 2010, the morning after a review of the book appeared in Maria Miller’s Homeschool Math Blog and in her email newsletter.

Today, the Amazon sales rank was 113,113. Cool number, eh? Reminds me of an arithmetrick:

  • Take a three‑digit number abc. Then, write it twice to make a six‑digit number abc,abc. (For instance, if you chose 113, then your six‑digit number would be 113,113.)
  • I’m feeling lucky, so divide by 7.
  • Hmm… I’m not feeling quite as lucky now, so divide by 11.
  • Uh-oh. I no longer feel lucky at all. Divide by 13.
  • Check your result. Should be the three‑digit number you started with, abc.

The following joke is a hint to why this trick works, in case you haven’t already figured it out:

Teacher: Can you find the prime factorization of 1,001?
Student: I didn’t even know it was lost!

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1 Comment Add your own

  • 1. xander  |  December 3, 2010 at 8:51 am

    So, clearly, there was going to be a trick at the end. I’ve seen these kinds of problems before, and I like to think that I am a smart cookie (though my wife calls me a smart Alec)! We started by dividing by 7, so, without bothering to read on, I went about proving that abc,abc is divisible by 7.

    Having done that, I got to the next step. Division by 11 is easy! \o/

    So, my number is divisible by 77. Cool. That’s kind of nifty. On to the last step. Division by 13, eh?

    Wait a minute…

    7x11x13 = 1,001.

    Crap.

    Now I feel dumb.😦

    Remember to read to the end of the problem before answering, xander! D’oh!

    Reply

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About MJ4MF

The Math Jokes 4 Mathy Folks blog is an online extension to the book Math Jokes 4 Mathy Folks. The blog contains jokes submitted by readers, new jokes discovered by the author, details about speaking appearances and workshops, and other random bits of information that might be interesting to the strange folks who like math jokes.

MJ4MF (offline version)

Math Jokes 4 Mathy Folks is available from Amazon, Borders, Barnes & Noble, NCTM, Robert D. Reed Publishers, and other purveyors of exceptional literature.

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