Tom needs to buy a new gaming system.  He trades in his super Nintendo for an original NES.  The SNES is worth $150 and the store gives him 80% of that value.  He gives $80 and gets back $10 change and a game worth $30.  How much was the NES on sale for?
He traded in the SNES for 150*.8=$<T>multiply(150, 0.8)=120.0
Including the money he gave, he spent 120+80=$<T>add(120, 80)=200
He got 10+30=$<T>add(10, 30)=40 in change
So the NES cost 200-40=$<T>subtract(200, 40)=160
