If A can complete a work in 15 days and B can complete it in 25 days, how long will it take for A to finish the remaining work if they worked together for 5 days?
Answer: C. 20 days
After 5 days, A and B complete 5(1/15 + 1/25) = 5(1/10) = 1/2 of the work. A needs to complete the remaining 1/2 work alone, which will take 15 days.