A train passes two persons walking in the same direction at a speed of 3 kmph and 5 kmph respectively in 10 seconds and 11 seconds respectively. The speed of the train is

Answer: Option C

1^st method:
Let the speed of the train be S. And length of the train be x.
When a train crosses a man, its travels its own distance.
$$\begin{array}{rl}& \text{According to question};\\ & \frac{x}{\left(s-3\right)\times \frac{5}{18}}=10\\ & \text{or},18x=50\times s-\mathrm{150.....}(\text{i})\\ & \text{and}\\ & \frac{x}{\left(x-5\right)\times \frac{5}{18}}=11\\ & 18x=55\times s-\mathrm{275......}(\text{ii})\\ & \text{Equating equation }\left(\text{i}\right)\text{ and }\left(\text{ii}\right)\\ & 50\times s-150=55\times s-275\\ & \text{or},5\times s=125\\ & \text{or},s=25\text{kmph}\end{array}$$