Description for Correct answer:
A's 1 day's work = 1 / 6
B's 1 day's work = 1 / 12

(A + B)'s 1 day's work =\( \Large\frac{1}{6} + \frac{1}{12} \)

\( \Large\frac{2 + 1}{12} = \frac{3}{12} = \frac{1}{4}\)

Hence, both will complete the work in 4 days.