A Great Day for a Math Trick

July 11, 2013 at 7:11 am 5 comments

Today is 7/11/13, and boy, have I got a great math trick for today! You’ll likely need a calculator.

  1. Multiply your age by 12.
  2. Now add the age of your spouse/brother/sister/friend/uncle/aunt/whomever.
  3. This should yield a three-digit number. Now, divide by 7.
  4. Then, divide by 11.
  5. Then, divide by 13.
  6. The result should be a number of the form 0.abcdef…, with a 0 and a decimal point in front of a long string of digits. Add the first six digits after the decimal point.

Here’s the cool part. I don’t know your age, nor do I know the age of your spouse, brother, sister, friend, uncle, or aunt. But I do know that after you completed those steps, the result was 27.

Pretty cool, eh?

There are myriad math tricks of this ilk, but this one is my favorite. It’s based on a trick I learned from Art Benjamin, though I think the one above has more panache than his original. Decide for yourself.

  1. Choose a number from 1 to 70, and then divide it by 7.
  2. If your total is a whole number (that is, no digits after the decimal point), divide the answer by 7 again.
  3. Is there a 1 somewhere after the decimal point? I predict that the number after the 1 is 4. Am I right?
  4. Now add up the first six digits after the decimal point.

Just as with the trick above, the result will always be 27.

Regardless of which trick you prefer, have a happy 7/11! And if you’ve got a few hours to kill, you can try to solve the 7‑11 problem.

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Being All Inclusive How Cool is This?

5 Comments Add your own

  • 1. John Golden  |  July 11, 2013 at 11:12 am

    Love this! Added it to my Google spreadsheet of number tricks, http://bit.ly/NumberTricks.

    • 2. venneblock  |  July 13, 2013 at 10:28 pm

      Nice set of tricks, John. Thanks for including mine.

  • 3. Sparky Teaching  |  July 11, 2013 at 11:33 am

    A great idea to bamboozle the kids with… Thanks. Hope you are well, sir! Just saw this on Twitter and realised it was you! (this is your NCTM illustration collaborator from the UK!)

    • 4. venneblock  |  July 13, 2013 at 10:30 pm

      No bamboozling, Paul… just educating! They are mystified when they hear that I could predict the result. That’s why we spend some time talking about how I was able to do it.

      And you didn’t need to tell me who you are — I recognize Sparky Teaching!

  • 5. A Date of Good Luck | Math Jokes 4 Mathy Folks  |  November 7, 2013 at 11:54 am

    […] which are the same numbers used in my favorite math trick. I shared that trick on 7/11/13 in A Great Day for a Math Trick — which, honestly, was the best day ever to share it (unless you’re in Europe, in […]


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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.

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