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?

Answer: Option B

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