Question

A train 300 metre long crosses a lamppost in 25 seconds. Calculate the speed of the man running in opposite direction who takes 20 seconds to cross the train.

1. 15 m/s
2. 5 m/s
3. 10 m/s
4. 2 m/s
5. 3 m/s

Answer 1

Correct Answer - Option 5 : 3 m/s

Given:

Length of train = 300 m

Time taken to cross the lamppost = 25 seconds

Time taken to cross man running in the opposite direction = 20 seconds

Concept:

Relative speed while going in the opposite direction = sum of the speeds of objects

Distance covered in crossing post or man is the length of the train

Formula:

Speed = distance/time

Calculation:

Speed of train = 300/25 = 12 m/s

Let the speed of man be x m/s

Time taken to cross the train = 20s = distance of train/relative speed 

Relative speed = 12 + x 

20 = 300/(12 + x)

12 + x = 15

Speed of man = 15 – 12 = 3 m/s

∴ Speed of the running man is 3 m/s