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, A's work rate is 1/30. Together they work at 1/20, so B's rate is 1/20 - 1/30 = 1/60. Therefore, B will take 60 days to complete the work alone.