They each give $10 to the waiter. The waiter goes to the cashier who notices that the waiter had made a mistake in tallying the tab. The correct total is $25. The cashier gives the waiter five $1 coins in change (they don't print $1 bills anymore).

On the way back to the table, the waiter ponders on how he is going to split $5 three ways.

No problem, he thinks. "They think the total is $27. I will give them each $1 in change and I'll keep the remaining $2. Problem solved."

Here is the math question.

Each customer paid $9, times three equals $27. The waiter kept $2, equals $29. What happened to the missing $1?

(This one works best after your friends have had a few drinks.)