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Simple Math Telepathy

Ask your audience to follow the following instructions
  • Rearrage the digits in the written number
  • Find the difference between the two numbers
  • Cross out one digit from the answer
  • Read out the remaining digits in any order

Without peeking into their worksheet, you give them the crossed out digit.

Trick: Rearranging the digits in the number does not change its digit sum. Finding the difference between them will produce a number with a a digit sum of zero ( a multiple of 9)

So add the remaining digits read out to you mentally and subtract the sum from the next higher multiple of nine. The result is the number crossed out. If the sum is a multiple of 9, then the number crossed out must be a nine or a zero.

When adding the remaining digits, you can employ the method of casting out 9s.
Practice this trick with your parents or friends.
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