If 6 persons working 8 hours a day earn Rs. 8400 per week, then 9 persons working 6 hours a day will earn per week ?

Answer: Option B

Let the weekly earning be Rs. x
More persons , More earning (Direct proportion)
Less working hours, Less earning (Direct proportion)
$$\begin{array}{c}\text{Persons 6}:9\text{hfill}\\ \text{Working hours 8}:6\text{hfill}\end{array}\}::8400:x$$
$$\begin{array}{rl}& \therefore 6\times 8\times x=9\times 6\times 8400\\ & \iff x=\frac{\left(9\times 6\times 8400\right)}{\left(6\times 8\right)}\\ & \iff x=9450\end{array}$$