A can do a work in 18 days and B can do it in 24 days. If they work together for 6 days, how much work is left?
Answer: A. 1/3
A's rate is 1/18 and B's rate is 1/24. Together they complete 1/18 + 1/24 = 7/72 of the work per day. In 6 days, they complete 6 * 7/72 = 7/12 of the work, leaving 5/12.