If 4/5 of an estate be worth Rs. 16800, then the value of 3/7 of the estate is = ?

Answer: Option A

Let the value of the estate be Rs. x
$$\begin{array}{rl}& \text{Then}\frac{4}{5}\text{ of }x=16800\\ & \iff x=\left(\frac{16800\times 5}{4}\right)\\ & =21000\\ & \iff \frac{3}{7}x=\left(\frac{3}{7}\times 21000\right)\\ & \text{ = 9000 }\end{array}$$