A and B can complete a work in 15 days and 20 days respectively. If they work together for 5 days, how much work is left?
Answer: A. 1/3
A's rate is 1/15 and B's rate is 1/20. Together, they complete 1/15 + 1/20 = 7/60 of the work per day. In 5 days, they complete 5 * 7/60 = 7/12 of the work, leaving 5/12 of the work remaining.