A and B can complete a work in 20 days together. If A alone can do it in 30 days, how long will B take to complete the work alone?
Answer: A. 40 days
If A takes 30 days, then A's work rate is 1/30. Together they complete it in 20 days, so their combined rate is 1/20. Thus, B's rate is 1/20 - 1/30 = 1/60, meaning B takes 60 days.