ANSWER--> WHERE DID THE DOLLAR GO? 

It is all in how the question is asked. 

When the friends paid $10 dollars, they had paid $30 in total.

When the Cashier gave $5 dollars to the Waiter, the 3 friends had paid $25 to the Cashier and $5 to the Waiter. 

When the Waiter returns 3 dollars, the 3 friends had paid $25 to the Cashier and $2 to the Waiter. $25+$2 = $27 = 3 x $9.


We 💗 Maths