A can do $\dfrac{1}{4}$ of a work in 5 days and B can do $\dfrac{1}{3}$ of the same work in 10 days. Find the number of days in which both working together will complete the work.

A's 5 days work = $\dfrac{1}{4}$

A's 1 day work = $\dfrac{1}{4 \times 5}$

= $\dfrac{1}{20}$

B's 10 days work = $\dfrac{1}{3}$

B's 1 day work = $\dfrac{1}{3 \times 10}$

= $\dfrac{1}{30}$

(A + B)'s 1 day work = $\dfrac{1}{20} + \dfrac{1}{30}$

= $\dfrac{(3 + 2)}{60}$

= $\dfrac{5}{60}$

= $\dfrac{1}{12}$

A and B can do the work in $\dfrac{12}{1}$ days

Hence, A and B working together can complete the work in 12 days.