C can complete a task in 12 days, while D can do it in 18 days. If they work together for 6 days, how much of the work is left?
Answer: A. 1/3
C and D together complete 1/12 + 1/18 = 5/36 of the work in one day. In 6 days, they complete 30/36 = 5/6 of the work, leaving 1/6 of the work remaining.