If A can complete a work in 20 days and B can complete it in 30 days, how long will it take for A to finish the work alone after working with B for 5 days?
Answer: B. 10 days
In 5 days, A and B together complete 1/20 + 1/30 = 1/12 of the work. The remaining work is 11/12, which A can complete in 11/12 * 20 = 15 days.