Three diners share a meal advertised at $30. At the end of the meal, each diner pays $10 to the waiter.
In fact, there is a $5 discount on the meal that day, but the crooked waiter announces that the discount is only $3. He then hands a dollar back to each of the three diners and pockets the remaining $2.
The three diners are none the wiser for the swindle, being happy enough to get a discount. So each diner paid $10 and got back a dollar, i.e., each diner paid $9. In total, they thus paid $27. The waiter took $2. That makes $29.