She percentage of change would be the percentage of reduction which is a 25% change. I found this out by doing 20-15 = 5 so 5/20 simplified fully would be 1/4 and 1/4 as a percentage is simply 25%. I can now check my answer by doing 25% of $20 which is $5 and take this away from the original amount. So 20-5 is therefore $15, the correct answer! ;)