Game 23

Here's a trick that will surely amaze students who have not yet discovered the real power of algebra:

Face the students with the blackboard behind you. Ask for one volunteer to write a number between 50 and 100 on the board. Ask him/her to add 76 to the number. Next, ask him/her to add the first digit of the result to the last two-digit number. Finally, subtract the result from the original number he wrote on the board. Surprise the entire class by "guessing" that the final number on the board is 23.

Example:
original number = 53
add 76 to 53 = 129
add 1 to 29 = 30
subtract 30 from 53 = 23

* I discovered recently that there is a coming movie with the title "Game 23".