last month you spent $20 on clothing. This month you spent 180% of what you spent last month. Set up a proportion to model this situation.

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%