## 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.

- Multiply your age by 12.
- Now add the age of your spouse/brother/sister/friend/uncle/aunt/whomever.
- This should yield a three-digit number. Now, divide by 7.
- Then, divide by 11.
- Then, divide by 13.
- 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.

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

Entry filed under: Uncategorized. Tags: 11, 7, Art Benjamin, decimal, digits, math, trick.

1.John Golden | July 11, 2013 at 11:12 amLove this! Added it to my Google spreadsheet of number tricks, http://bit.ly/NumberTricks.

2.venneblock | July 13, 2013 at 10:28 pmNice set of tricks, John. Thanks for including mine.

3.Sparky Teaching | July 11, 2013 at 11:33 amA 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 pmNo 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 […]