Figure this out.
3 guys eat a meal and are charged $30, they split it 3 ways and pay the waiter $30, he takes it to the till and finds out that the meal was acutally $25.
The waiter returns to the table with the $5 change in $1 bills. The 3 guys take $1 each and give the waiter the other $2 for his tip.
That now means each guy has paid $9 (the $10 they paid, less the $1 change) for his meal and the waiter has $2.
So 3 guys x $9 = $27 add the waiters $2 and you get $29. Where did the other $1 go?