What is the percent decrease 20 miles to 11 miles

I think it's 45% because you'd have to convert 20 to 100 by multiplying 20 by 5 and if you multiply 20 by 5, you'd have to do the same to 11.And 11 ×5 =55, and 100-55=45
So,it's 45%
(there's most likely a chance that my answer is wrong because I think is a middle school question and I'm in 5th grade)

The answer is: a = about 63.59ft2 for 15