The Magic Number

Try this mind-readng trick on a friend. Have him choose a number from 1 to 100 and multiply it by 99. Then ask him to add up the numbers in his answer. Regardless of the number he chooses, the final answer will always be 18.

For example: suppose your friend chooses the number 11.

11 x 99 = 1089

1 + 0 + 8 + 9 = 18