If A can finish a work in 24 days and B can finish it in 36 days, how long will it take for A to finish the work alone after working with B for 12 days?
Answer: B. 12
In 12 days, A and B together complete 12*(1/24 + 1/36) = 12*(5/72) = 10/12 of the work. A will then need to finish 2/12 = 1/6 of the work, which takes 4 days.