30 men can do a piece of work in 16 days. How many men would be required to do the same work in 20 days ?

Answer: Option B

Let the required number of men be x
More days, Less men (Indirect proportion)
$$\begin{array}{rl}& \therefore 20:16::30:x\\ & \iff 20x=16\times 30\\ & \iff x=\left(\frac{16\times 30}{20}\right)\\ & \iff x=24\end{array}$$