A can do a piece of work in 30 days, B can do it in 20 days, and C can do it in 15 days. If they work together, how long will it take to complete the work?
Answer: B. 6 days
Their combined work rate is 1/30 + 1/20 + 1/15 = 1/6. Therefore, they will complete the work in 6 days.