A and B can together do a piece of work in 15 days. B alone can do it in 20 days. In how many days can A alone do it ?

A. 40 days
B. 50 days
C. 60 days
D. 70 days

(A+B)’S 1 day’s work = [1/15−1/20]=1/60
A alone can finish it in 60 days