Magic No.9

You've got to do this with a friend! 

1. Ask a friend to work a subtraction problem on a calculator.  
2. Get her to tell you either the first digit or the last digit of the answer.  
3. 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

(099)

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.

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Let the hands calculate multiplication of 9! 

 

Awesome isn't it? :)