36 men can complete a piece of work in 18 days. In how many days will 27 men complete the same work?

Answer: Option D

Let the required number of days be x.
Less men, More days (Indirect Proportion)
$$\begin{array}{rl}& \therefore 27:36::18:x\\ & \Rightarrow 27\times x=36\times 18\\ & \Rightarrow x=\frac{36\times 18}{27}\\ & \Rightarrow x=24\end{array}$$