Get the basics right for Police recruitment exam

This article is in continuation to the last article on preparation for the Sub-Inspector of Police recruitment exam. TRAINS: It is a part of Time and Distance. The train based questions depend on time, distance and speed. Before the question, first we will discuss basics in trains. If a train has to cover a standing […]

This article is in continuation to the last article on preparation for the Sub-Inspector of Police recruitment exam.

TRAINS:
It is a part of Time and Distance. The train based questions depend on time, distance and speed. Before the question, first we will discuss basics in trains.

If a train has to cover a standing person or a tree or pole, then train has to cover its own length.

Here
Distance = Length of the train
If a train has to cross a platform or a bridge, then train has to cover, the sum of lengths of train and platform or bridge.
Distance = Sum of lengths of train and platform or bridge

If two trains are running same direction, when they cross each and other, the distance and relative speeds are as follow:

Distance = Sum of lengths of two trains
Relative Speed = Difference between speeds of two trains
So, the time taken by the trains to cross each other = Distance/Speed
However, if two trains are running in opposite direction, when they cross each and other, the distance and relative speeds are as follow.

Distance = Sum of lengths of two trains
Relative Speed= Sum of speeds of two trains
So, the time taken by the trains to cross each other = Distance/Speed

1. A train passes a station platform in 36 seconds and a man standing on the platform in 20 seconds. If the speed of the train is 54 km/hr, what is the length of the platform?

Solution:
Speed = 54 km/hr = 54 ×5/18 = 15 m/s
Length of the train = 20 × 15 = 300 m
Let the length of the platform is ‘x’ m
Then (x 300)/36=15
x 300 = 540
x = 540 – 300 = 240 m
Therefore the length of the platform = 240 m

2. Say, 300 m and 400 m lengths of two trains are running at 88 km/hr and 92 km/hr respectively. If they are running in opposite direction on parallel railway tracks, then at what time they cross each and other?

Solution:

Distance = 300 400 = 700 m
Relative Speed = 88 92 = 180 km/hr = 180 × 5/18 = 50 m/s
Time = 700 / 50 = 14 sec.

3. About 240 m length of train is running at 80 km/hr. Then at what time it will cross a girl, who is running 8 km/hr same direction?

Solution:

Distance = 240 m
Relative Speed = 80 – 8 = 72 km/hr = 20 m/s
Time = 240 / 20 = 12 sec

4. Say, 350 m length of train is running at 82 km/hr. Then at what time it will cross a boy, who is running 8 km/hr opposite direction?

Solution:

Distance = 350 m
Relative Speed = 82 8 = 90 km/hr = 25 m/s
Time = 350/ 25 = 14 sec

Banda Ravipal Reddy
Director, SIGMA
Sai Institute of General Mental Ability
Hyderabad
Ph. 9491614131

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