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

(A+B)together do the work in 30 days.
$$\therefore$$ (A+B)'s 1 day's work=$$\frac{1}{30}$$
$$\therefore$$ (A+B)'s 20 days' work=$$\frac{20}{30}$$$$=\frac{2}{3}$$
Remaining Work=$$1-\frac{2}{3}$$
=$$\frac{1}{3}$$
$$\because$$ Time taken by A in doing $$\frac{1}{3}$$ work=20 days
$$\therefore$$ Time taken by A to complete the work=$$20\times3$$
=$$60$$days
Hence, Option A is correct.