Answer with Explanation:
Ratio of rates of working of A and B = 2 : 1.
So, ratio of times taken = 1 : 2.
B's 1 day's work = 1/12 .
Therefore A's 1 day's work = 1/6 ; (2 times of B's work)
(A + B)'s 1 day's work = (1/6 + 1/12) = 3/12 = 1/4 .
So, A and B together can finish the work in 4 days.