A's 1 day's work = 1 / 12

B's 1 day's work = 1/ 24

(A + B)'s 1 day's work = \( \Large\frac{1}{12} + \frac{1}{24} \)

=\( \Large\frac{2 + 1}{24} = \frac{3}{24} = \frac{1}{8}\)

(A + B) will finish the work in 8 days.