A and B can do a piece of work in 12 days; B and C in 15 days; C and A in 20 days. A alone can do the work in__________?
A. 20 days
B. 30 days
C. 40 days
D. none of these
A and B can do a piece of work in 12 days; B and C in 15 days; C and A in 20 days. A alone can do the work in__________?