There are two concentric circles with radii 8 and 4. If the radius of the outer circle is increased by 25% and the radius of the inner circle decreased by 25%, by what percent does the area between the circles increase?  Options:
A) 75
B) 80
C) 85
D) 90
E) 95
The area of a circle is piR^2, where R is the radius.   The area of the big circle is 64pi.   The area of the small circle is 16pi.   The area A1 between the circles is 48pi.   When the big circle's radius increases, the new area is 100pi.   When the small circle's radius decreases, the new area is 9pi.   The area A2 between the circles is 91pi.   The ratio of A2/A1 is 91/48 = 1.9 which is an increase of 90%.   The answer is 90.
<result>D</result>
There are two concentric circles with radii 8 and 4. If the radius of the outer circle is increased by 25% and the radius of the inner circle decreased by 25%, by what percent does the area between the circles increase?