A and B can do a piece of work in 12 days and 18 days respectively. If they work together for 6 days, how much work is left?
Answer: A. 1/3
A's work rate is 1/12 and B's is 1/18. Together they complete 1/12 + 1/18 = 5/36 of the work in one day. In 6 days, they complete 6 * 5/36 = 5/6 of the work, leaving 1 - 5/6 = 1/6 of the work left.