A takes 3 hours more than B to walk 'd' km. If A doubles his speed, then he can make it in 1 hour less than B. How much time (in hours) does A require to walk 'd' km?

Answer: Option C

Let the speed of the A be a and B be b
According to the question,
$$\frac{\text{d}}{\text{a}}-\frac{\text{d}}{\text{b}}=3----\left(\text{i}\right)$$
Again,
$$\frac{\text{d}}{\text{b}}-\frac{\text{d}}{2\text{a}}=1----\left(\text{ii}\right)$$
By adding equation (i) and (ii) we get,
$$\begin{array}{rl}& \frac{\text{d}}{2\text{a}}=4\\ & \Rightarrow \frac{\text{d}}{\text{a}}=4\times 2\\ & \Rightarrow \frac{\text{d}}{\text{a}}=8\end{array}$$
So, it will take 8 hours by A to cover the distance d
∴ A requires 8 hours to walk 'd' km.