A works twice as fast as B. If both of them can together finish a work in 12 days, then in how many days can B alone do it?
Answer: D
Let the efficiency of B be x units/day. Then the efficiency of A is 2x units/day. Their combined efficiency is 3x units/day. Total work = Efficiency * Time = 3x * 12 = 36x units. Time taken by B alone = Total work / B's efficiency = 36x / x = 36 days.