
Shortcut Trick
Let Train length be 3 units, then Platform length is 1 unit.
Total distance = 3 + 1 = 4 units.
Total distance = Speed × Time = 25 × 20 = 500 m.
4 units = 500 m ⇒ 1 unit = 125 m.
Train length = 3 units = 3 × 125 = 375 m.
∴ The correct answer is 375 m.

Alternate Method
Given:
Speed (S) = 25 m/s, Time (T) = 20 seconds, Platform (P) = 1/3 of Train (L).
Formula Used:
Distance = Speed × Time; Total Distance = Length of Train + Length of Platform

Calculations:
⇒ Let Train Length = L
⇒ Platform Length = L/3
⇒ Total Distance = L + L/3 = 4L/3
⇒ 4L/3 = 25 × 20
⇒ 4L/3 = 500
⇒ L = (500 × 3) ÷ 4
⇒ L = 125 × 3 = 375 m
∴ The correct answer is 375 m.

Additional Information
Relative Speed
When two objects move in the same direction, relative speed is S1 − S2. In opposite directions, it is S1 + S2.
Unit Conversion
To convert km/hr to m/s, multiply by 5/18. To convert m/s to km/hr, multiply by 18/5.
Object Crossing Rule
When a train crosses a pole or a man, distance = length of train. When it crosses a bridge or platform, distance = train length + bridge/platform length.