Question: a can do a piece of work in 60 days?
Answer: A can do (1/60)th of the work in a day. So in 15 days, A does 15*1/60 = 1/4th of the work. The remaining 3/4th of the work is done by B in 30 days. So the entire work would be done by B in 30*4/3 = 40 days.