So,
The percent discount is the amount of discount divided by the original price. In formula that is:

The discount is the difference between the original price and the sale price.
40 - 25 = $15
The discount is $15.
Divide the discount by the original price.

To convert from decimal form to percent form just move the decimal point two places to the right and add a percent sign.
0.375 --> 37.5 --> 37.5%
So the percent discount on the sweater is 37.5%.