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

Answer: Option C

Let the train's speed = x km/hr
  $$\because$$ When train will cross a man then it covers only its length.
$$\begin{array}{rl}& \therefore \left(x-3\right)\times \frac{10}{60}=\left(x-5\right)\times \frac{11}{60}\\ & x=25\text{ km/hr}\end{array}$$
Note :
Products of time × speed is always subtracted if both the men are running in the same direction and the products of time × speed is added only if the men are running opposite direction.
⇒ Here train's direction is not considered.
But attention please ⇒ Always divided by the difference of time.