It cost $4 to enter the fair. each ride costs $2.50. you have $21.50 in your walket. how many rides can you go on?

You can go on 7 rides.

So first you would subtract 21.50 - 4.00 = 17.50

then you would add 2.50 until you can't spend anymore so the answer is that you would only be able to go on 4 rides so you would use up all your money