A's 1 day's work = 1/ 4
B's 1 day's work= 1/12 (A + B)'s s 1 day's work = \( \Large\frac{1}{4} + \frac{1}{12} \) \( \Large\frac{3 +1}{12} = \frac{4}{12} = \frac{1}{3} \) Days required by A and B together to do the work
= 1 / (A + B)'s 1 day's work = 3 days