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? Let's be accurate as possible.
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.
The answer: 625.