C can finish a work in 20 days, while D can finish it in 25 days. If they work together, how long will it take them to finish the work?
Answer: B. 12 days
C's rate is 1/20 and D's is 1/25. Together, they complete 1/20 + 1/25 = 9/100 of the work per day, so they will finish in 100/9 days, approximately 11.11 days, which rounds to 12 days.