The correct answer is **(4) 50 metres**

### Solution

Let the length of train be x km

Let the speed of the train be y km/hr

2 kmph = (2 x 5/18) m/sec = 5/9 m/sec.

4 kmph = (4 x 5/18) m/sec = 10/9 m/sec.

According to the given condition

x/(y-2) = 9/(60*60)

x/(y-4) = 10/(60*60)

On combining both the equations we get

9y – 3600x = 18

and 10y – 3600x = 40

y-400x = 2

and y – 360x = 4

Therefore

40x = 2

x = (2/40 *1000)m

= 50 metres