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