A and B can do a piece of work in 20 days and 30 days respectively. If they work together for 10 days, how much of the work is left?
Answer: A. 1/3
A's rate is 1/20 and B's rate is 1/30. Together they complete 1/20 + 1/30 = 1/12 of the work per day. In 10 days, they complete 10/12 = 5/6 of the work, leaving 1/6.