A and B can complete a work in 12 days and 16 days respectively. If they work together for 4 days, how much work is left?
Answer: A. 1/3
A's rate is 1/12 and B's is 1/16. Together, they complete 1/12 + 1/16 = 7/48 of the work per day. In 4 days, they complete 28/48 = 7/12 of the work, leaving 5/12.