Adapted from Mathemagic by Don Fraser, page 29 - www.amazon.com/Mathemagic-Don-Fraser/dp/0201133679
Ask the audience for a 3-digit number.
Have the audience divite the 3-digit number by 9, and circle the remainder.
To obtain the remainder within seconds, add up the 3 digits. If the result is less than 9, that is the remainder. If the result is greater than 9, sum the digits, and continue summing up the digits until you get a number that is between 0-8; that result will be the remainder of dividing the 3-digit number by 9. If the result is 9, the remainder is 0. For example, the sum of the digits of 611 is 8, so 611 divided by 9 has a remainder of 8. The sum digits in 876 is 21 and the sum of the digits in 21 is 3, so the remainder of 876 divided 9 is 3.
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