C can complete a task in 10 days, while D can complete it in 15 days. If they work together for 5 days, how much of the work is left?
Answer: B. 1/4
C's rate is 1/10 and D's is 1/15. Together, they complete 1/10 + 1/15 = 1/6 of the work per day. In 5 days, they complete 5/6 of the work, leaving 1/6 remaining.