There are two concentric circles with radii 6 and 4. If the radius of the outer circle is increased by 50% and the radius of the inner circle decreased by 25%, by what percent does the area between the circles increase? Choose the correct choice from the following.
A) 160	B) 210	C) 260	D) 310	E) 360

Rationale: "the area of a circle is pir ^ 2 , where r is the radius . the area of the big circle is 36 pi . the area of the small circle is 16 pi . the area a 1 between the circles is 20 pi . when the big circle ' s radius increases , the new area is 81 pi . when the small circle ' s radius decreases , the new area is 9 pi . the area a 2 between the circles is 72 pi . the ratio of a 2 / a 1 is 72 / 20 = 3.6 which is an increase of 260 % . the answer is c ."
Answer: C