You can lower the price by 20% if you buy more than fifteen units of iPhone cases. If you pay $500 to buy 18 units, what is the original price?
If you buy 18 units, the percentage of the total price drops to 100% - 20% = 80%
If 80% is equal to $500, then 100% is equals 100%/80% * $500 = $625
#### 625