A and B can do a work in 12 days and 18 days respectively. If they work together for 6 days, how much work is left?
Answer: C. 1/2
In 6 days, A and B together complete 1/12 + 1/18 = 5/36 of the work. Therefore, the remaining work is 1 - 5/36 = 31/36, which simplifies to 1/2 of the total work.