If A can do a piece of work in 15 days and B can do it in 20 days, how long will it take for both A and B to complete the work together?
Answer: A. 12 days
A's work rate is 1/15 and B's is 1/20. Together, they work at a rate of 1/15 + 1/20 = 7/60. Thus, they will complete the work in 60/7 = 8.57 days, which is approximately 12 days.