A can do a piece of work in 12 days, B can do the same work in 15 days. If they work together for 3 days, how much of the work is left?
Answer: B. 1/3
In 3 days, A and B together complete 3*(1/12 + 1/15) = 3*(5/60 + 4/60) = 3*(9/60) = 27/60 = 9/20 of the work. Therefore, 1 - 9/20 = 11/20 of the work is left.