The Secret of 73 and 137
Let’s have a game.
For this trick, secretly write 73 on a piece of paper, fold it up, and give to an unsuspecting friend.
Now have your friend select a four-digit number whatever (say, 3125) and enter it twice into a calculator. (31253125)
Announce that the number is divisible by 137 and have him verify it on his calculator.
Next, announce that he can now divide by his original four-digit number. After he has done so, dramatically command him to look at your prediction on the paper. It will match his calculator display: 73!