If A can finish a work in 24 days and B can finish the same work in 36 days, how long will it take for them to finish the work if A works alone for 10 days and then B joins him?
Answer: B. 20 days
A completes 10/24 of the work in 10 days. The remaining work is 14/24. A and B together can complete 1/24 + 1/36 = 5/72 of the work per day, taking 14/5/72 = 20 days to finish the remaining work.