Working 8 hours a day, 12 men can do a work in 30 days. Working 4 hours a day, 18 men can do work in ?

Answer: Option B

Let the required number of days be x
Less working hours, More days (Indirect Proportion)
More men, Less days (Indirect Proportion)
$$\begin{array}{c}\text{Working hours 4}:8\text{hfill}\\ \text{Men 18}:12\text{hfill}\end{array}\}::30:x$$
$$\begin{array}{rl}& \therefore 4\times 18\times x=8\times 12\times 30\\ & \iff x=\frac{\left(8\times 12\times 30\right)}{\left(4\times 18\right)}\\ & \iff x=40\end{array}$$