AMC 8 1986 Problem 18

A rectangular grazing area is to be fenced off on three sides using part of a $100$ meter rock wall as the fourth side. Fence posts are to be placed every $12$ meters along the fence including the two posts where the fence meets the rock wall. What is the fewest number of posts required to fence an area $36$ m by $60$ m?

$\text{(A)}\ 11 \text{(B)}\ 12 \text{(C)}\ 13 \text{(D)}\ 14 \text{(E)}\ 16$