For a 20% decrease in the radius of a circle, what will be the percentage reduction in its area?

A 10%

B 20%

C 36%

D 400%

Correct Answer: Option C

If R is the original radius then the area of the circle is =πr2

If we decrease the radius by 20% the new radius is =.8r so the new area is=π .(8r)2 = .64πr2

Compute the percent of the new to the old: = 64πr2 / πr2=.64

The decrease then is 100 - 64 = 36%