C can finish a work in 20 days, while D can finish it in 30 days. If they work together for 10 days, how much of the work is left?
Answer: A. 1/3
C's work rate is 1/20 and D's 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 of the work remaining.