Here’s a trick you probably didn’t know about. All you need is a calendar, and an audience. Show off your magic and mental math skills, and surprise your friends. This is great for budding mathematical geniuses to showcase their abilities!

What to do:

Tell your friend to pick a group of nine numbers but make sure those numbers are in a 3 by 3 rectangle. Now surprise him by quickly adding those numbers, and telling him the answer!

The Trick:

Relax…you don’t have to add all those numbers in your head. To get the sum of those nine numbers, you just have to multiply the center number by 9. The trick works for any group of nine numbers.

Example:

In our case, the group of nine numbers is 14, 15, 16, 21, 22, 23, 28, 29, and 30 as shown in the above diagram.

The center number is 22.
Multiply that number by 9 (22 x 9)
The result is 198.

If you add the nine numbers, the sum is 198.
14 + 15 + 16 + 21 + 22 + 23 + 28 + 29 + 30 = 198.