7-11-13 Trick!
Mystify your friends with the blazing speed of your brain.
Issue this challenge to your friend: You can get the answer to a
complicated multiplication problem before they do, even though they will be
using a calculator.
Have them write down a three-digit number, such as 452 or 993, on a piece of
paper and give the paper to you. You give them a calculator.
Then have them:
1. Enter the three-digit number on the calculator.
2. Multiply the number by 7.
3. Multiply that number by 11.
4. And, finally, multiply their total by 13.
While they are busy calculating, you just write down the answer on the piece
of paper.
THE SECRET: All you do is write out the starting number twice!
So 452 would become 452,452 and 993 would become 993,993.
Do you get it?
This was taken from Uncle John's Book of Fun