Hobbies And Interests
Properties of 12345679
Multiplying nine by 12345679 = 111111111. You can exploit this fact by asking your audience to choose a number between one and nine. Have them multiply this number first by nine and then by 12345679 (this number is very easy to memorize). The resulting number will be their chosen number multiplied by 111111111. You can then identify their chosen number by choosing any one digit in the resulting number.
Properties of 1001
Have your audience choose a three-digit number and enter it in their calculators twice in a row (e.g., if they pick 123, they will enter 123123 into their calculators). This is the same as multiplying by 1001, but there is no need to tell your audience this. Since 1001 is equal to 7 x 11 x 13, having your audience divide by 11, seven and then their three-digit number will yield 13, which you can handily "predict" long before they actually obtain that number.
Properties of 10001
The number 10001 is equal to 137 multiplied by 73. To exploit this fact, have your audience pick a four-digit number and enter the number formed by repeating its' digits twice into their calculators. You can then have your audience divide their number by 137 and by their four-digit number. This will always leave them with 73.
The Golden Ratio and the Fibonacci Sequence
Have your audience choose any two whole numbers between one and 100 and place them in sequence. Then have them create a sequence of 25 numbers, determining each term by adding the two preceding numbers. For example starting numbers of one and one will yield the sequence (1,1,2,3,5,8,13...). The 25th number of the sequence divided by the twenty-fourth will amazingly yield the golden ratio, 1.618033989. Memorize this number and they will never know how you did the trick.