A and B together can finish a work in 30 days. They worked for it for 20 days and then B left the work. The remaining work was done by A alone in 20 days more. In how many days can A alone finish the work?

Let total work to be done = 60 units

A and B together can finish the work in 30 days

=> (A+B)'s 1 day's work = $$\frac{60}{30}=2$$ units/day

Work done by them in 20 days = $$2 \times 20=40$$ units

Work left = $$60-40=20$$ units

This work is completed by A alone in 20 days

=> A's efficiency = $$\frac{20}{20}=1$$ unit/day

$$\therefore$$ Time taken by A alone to finish the work = $$\frac{60}{1}=60$$ days

=> Ans - (D)