Let total work to be done = 5y units Let B's efficiency = x units/day ⇒ A's efficiency = 3x units/day Work done by (A+B) together in 1 day = (x+3x)=4x units/day ⇒ Work done in 9 days =9×4x=36x units --------(i) Also, according to ques, work done in 9 days =
2
5
×5y=2y units -----(ii) ⇒ 36x=2y ⇒
y
x
=
36
2
=18 --(iii) ∴ Number of days by which B can do the work alone =