A's 1 day's work A = 1 / x

B's 1 day's work B = 1 / 3x

(A + B)'s 1 day's work = \( \Large\frac{1}{x} + \frac{1}{3x} = \frac{4}{3x} \)

and given one day work of both A

and B = 1/12

\( \Large\frac{4}{3x} = \frac{1}{12} = > 3x = 48 = > x = 16 \)