The task is to generate the correct answer for a given math problem.

Problem: the curved surface of a sphere is 64 π cm 2 . find its radius ?
4', 'd ) 9', 'e ) 7'

Problem: 30 pens and 75 pencils were purchased for 750 . if the average price of a pencil was 2.00 , find the average price of a pen .
20

Problem: what is the rate percent when the simple interest on rs . 720 amount to rs . 180 in 4 years ?

6.25