A and B can together finish a work in 30 days. They worked on 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 and B can together finish a work= 30 days.

One day work of A and B $=\frac{1}{30}$

20 days work of A and B $=\frac{20}{30}$

Remaining work $=1-\frac{20}{30}=\frac{1}{3}$work

$\therefore \frac{1}{3}$work was done by A alone $= 20$ days

 $\therefore$Whole work was done by A alone $= 20x3 = 60$ days