A and B can together do a piece of work in 15 days. B alone can do it in 20 days. In how many days can A alone do it ?
30 days
40 days
45 days
60 days
Explanation:
(A + B)’s 1 day’s work = $\dfrac{1}{15}$ B’s 1 day’s work = $\dfrac{1}{20}$ ∴ A’s 1 day’s work = $\frac{1}{15}$-$\dfrac{1}{20}$ = $\dfrac{4-3}{60}$= $\dfrac{1}{60}$ ∴ A alone will do the work in 60 days.
(A + B)’s 1 day’s work = $\dfrac{1}{15}$ B’s 1 day’s work = $\dfrac{1}{20}$ ∴ A’s 1 day’s work = $\frac{1}{15}$-$\dfrac{1}{20}$ = $\dfrac{4-3}{60}$= $\dfrac{1}{60}$ ∴ A alone will do the work in 60 days.