A can do a work in 15 days and B can do the same work in 10 days. If they work together for 5 days, how much of the work is left?
Answer: A. 1/3
A's work rate is 1/15 and B's work rate is 1/10. Together, they complete 1/15 + 1/10 = 1/6 of the work per day. In 5 days, they complete 5/6 of the work, leaving 1/6 of the work remaining.