A and B can 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:
Answer: C
The work that (A+B) could have done in the remaining 10 days (30-20) was done by A in 20 days. So, \(10(A+B) = 20A \Rightarrow 10A+10B = 20A \Rightarrow 10B = 10A \Rightarrow A=B\). This means they have equal efficiency. If they together take 30 days, one person would take double the time, i.e., 60 days. So A alone can finish the work in 60 days.