A and B can finish a work in 12 days and 15 days respectively. If they work together for 4 days, how much of the work is left?
Answer: C. 1/3
In 4 days, A and B together complete 4*(1/12 + 1/15) = 4*(5/60 + 4/60) = 4*(9/60) = 3/5 of the work, leaving 2/5 of the work remaining, which is 1/3 of the total work.