- Ask a friend to work a subtraction problem on a calculator.
- Get her to tell you either the first digit or the last digit of the answer.
- You are now able to divulge the entire answer!
How to Do It
Here are all the possible answers when you subtract two 3-digit numbers as described.
99 198 297 396 495 594 693 792 891
Notice that the middle digit is always 9 and that the sum of the first digit and the last digit is 9. So just subtract what your friend tells you from 9 to get the missing digit.
Awesome isn't it? :)