5 persons can prepare an admission list in 8 days working 7 hours a day. If 2 persons join them so as to complete the work in 4 days, they need to work per day for ?

Answer: Option C

Let the number of working hours per day be x
More persons, Less working hours ( Indirect proportion)
Less days, More working hours (Indirect proportion)
$$\begin{array}{c}\text{Persons 7}:5\text{hfill}\\ \text{Quantity 4}:8\text{hfill}\end{array}\}::7:x$$
$$\begin{array}{rl}& \therefore 7\times 4\times x=5\times 8\times 7\\ & \iff x=\frac{\left(5\times 8\times 7\right)}{\left(7\times 4\right)}\\ & \iff x=10\end{array}$$