A and B together can do a work in 15 days whereas B alone can do same work in 20 days. In how many days can A alone do the work ?

60

A and B can do a work in 15 days

B alone can do the work in 20 days

Efficiency = Total work/Number of days

L.C.M of 15 and 20 = 60 = Total work

Efficiency of A and B together = 60/15 = 4 units/day

Efficiency of B alone = 60/20 = 3 units/day

Efficiency of A = (4 – 3) = 1 unit/day

Number of days taken by A alone = Total work/Efficiency = 60/1 = 60 days