Calendar Math Trick

This calendar math trick is yet another amazing math activity that lets kids (and adults) see just how cool math is!  Just grab a calendar from off the wall or wherever you can get one.

The Effect:

  • Have a friend circle a group of nine numbers.  Make sure the numbers are in a 3 by 3 rectangle like the picture shows here.
  • Tell your friend that you have the special ability to add all nine of the numbers circled in your head in a matter of seconds!
  • You give the correct answer and have them check it with a calculator.

The Secret:  All you have to do to get the sum of all nine numbers is to multiply the number in the center of the circled numbers by nine!  So for our example, the center number is 11. 

So you just need to multiply in your head, “9 x 11”.

For those of you who’ve memorized your multiplication facts, you know right away that the answer is 99.

HANDY TIP: To multiply by 9 quickly, just multiply by 10 then subtract your number. So to get 22 x 9 you multiply 22 x 10 =220 (easy!) and then subtract 22. With a bit of practice you can do this quickly in your head.

After you perform the trick once, perform it again, this time make it a little more interesting.  Challenge them to a race.  Say to your friend, “I will add the nine numbers in my head while you add them with the calculator, and I will get the answer first”.  You just perform it the same way as before – just multiply the middle number by 9 to get the answer.

They should be pretty impressed and if they are like most, will wonder how it was done.  Usually no one will figure out the secret. 

Now, how about a grand finale to impress them even more.  This time tell them to circle 20 numbers in a 5 by 4 rectangle – and you will add up all the circled numbers in your head in a couple of seconds! 

Take a look a the picture below.

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The Secret: Add the smallest and largest numbers that are circled and multiply their sum by 10.  And that's it.  Pretty cool, huh. 

So for the above example;

6 + 31 = 37 and then,

37 x 10 = 370

So the sum of all 20 numbers is 370!

Have fun with this trick.  No one will be able to figure out how you get the answer so fast!  If you liked this calendar math trick, we will be adding another real soon that is also really cool.

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