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Seventy Three
Thanks Bob K

Find a piece of paper, a pencil, a calculator and a friend

Write 73 on a piece of paper, fold it up, and hand it to your unsuspecting friend.

Ask your friend select a four-digit number and enter it twice into a calculator.
Example : 23452345 

Ask your friend to divide this number by 137.
Example : 23452345 / 137 = 171185

Now ask your friend to divide the answer by his original four-digit number
Example : 171185 / 2345 = 73

Ask your friend to look at the folded piece of paper.

You are a Math Magician!

Why this happens :
Entering a four-digit number twice (23452345) is equivalent to multiplying it by 10001.(2345 x 10001 = 23452345).
Since 10001 = 73 x 137, the eight-digit number will be divisible by 73, 137, and the original four-digit number.

:  Try more math tricks here.