39 persons can repair a road in 12 days, working 5 hours a day. In how many days will 30 persons, working 6 hours a day, complete the work?

Explanation:-

Let the required number of days bex.
Less persons, More days (Indirect Proportion)
More working hours per day, Less days (Indirect Proportion)
$$\left. \begin{gathered} \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\text{Persons }}\,30:39 \hfill \\ {\text{Working hour/day }}\,{\text{6}}:{\text{5}} \hfill \\ \end{gathered} \right\}::12:x$$
$$\eqalign{ & \therefore 30 \times 6 \times x = 39 \times 5 \times 12 \cr & \Rightarrow x = \frac{{\left( {39 \times 5 \times 12} \right)}}{{\left( {30 \times 6} \right)}} \cr & \Rightarrow x = 13 \cr}$$