请教：怎么对付估算题？

对于数学的估算题比较头疼，比如下面一道

MATH.1

Q7:

A circular mat with diameter 20 inches is placed on a square tabletop, each of whose sides is 24 inches long.  Which of the following is closest to the fraction of the tabletop covered by the mat?

A.     5/12

B.     2/5

C.     1/2

D.     3/4

E.      5/6

应该选D

请问大家有没有什么对付估算题的好方法呢？

感谢感谢！