A and B can together finish a work in 30 days. They worked together for 20 days and then B left. After another 20 days, A finished the remaining work. In how many days A alone can finish the job?

Question: A and B can together finish a work in 30 days. They worked together for 20 days and then B left. After another 20 days, A finished the remaining work. In how many days A alone can finish the job?

Solution:
(A + B)'s 20 days work = (1/30) × 20 = 2/3 part

Remaining work = 1 - 2/3 = 1/3 part

1/3 of part work is done by A in 20 days
∴ 1 part work is done by A in 20 × 3 days
= 60 days