A can do a piece of work in 15 days, but with the help of B, he can do it in 9 days. In what time can B do it alone?

22.5 days 18 days 21 days 20.5 days

22.5 days

A = 15 days, A + B = 9 days, ⇒ A + B = 5  (Efficiency) ⇒ 3 + B = 5, ⇒ B = 2, ⇒ Time required by B to complete work alone = \(\frac{45}{2}\) = 22.5 days,   ..(\(\frac{Work}{Efficiency}\) = Time)