This is a kind of juggling-like questions, if you think along with his thoughts, you will only feel puzzled, and a little stroke of thought, you can find the mystery. First of all, the $9 spent per person already includes the $2 hidden by the waiter (i.e., the discounted price of $25 + $2 hidden by the waiter = $27 = 3 × $9) Therefore, when calculating the composition of the $30 can not be counted in the $2 hidden by the waiter, but should be added to the $1 returned to each person. That is: 3 × 9 + 3 × 1 = 30 dollars exactly!