Ratio of efficiency of A and B = 2 : 1 Then, ratio of their time taking = 1 : 2 Hence, if B can complete the work in 12 days, then A in 6 days. Now, (A + B)'s 1 day's work =
1
6
+
1
12
=
3
12
=
1
4
So, A and B together can complete the work in 4 days.