First the absolute would decrease
20 - 15 = $5
Then the percent decrease is the absolute decrease divided by the the original price (part/whole).
percent decease = 5 / 20 = 0.25
Multiply and divide 0.25 to obtain percent.
percent decease = 0.25 = 0.25 * 100 / 100 = 25 / 100 = 25%