12 persons can do a piece of work in 4 days. How many persons are required to complete 8 times the work in half the time ?

Answer: Option D

Let the required number of persons be x
Less days , More persons (Indirect proportion)
More work , More persons (Direct proportion)
$$\begin{array}{c}\text{Days 2}:4\text{hfill}\\ \text{Work 1}:8\text{hfill}\end{array}\}::12:x$$
$$\begin{array}{rl}& \therefore 2\times 1\times x=4\times 8\times 12\\ & \iff x=\frac{\left(4\times 8\times 12\right)}{\left(2\right)}\\ & \iff x=192\end{array}$$