Shortcut Trick
Ratio of Time taken (Same direction : Opposite direction) = 72 : 36 = 2 : 1.
Ratio of Relative Speed (Same : Opposite) = 1 : 2 (since Distance is same).
Let speeds be VA and VB. ⇒ (VA − VB) / (VA + VB) = 1 / 2.
By Componendo and Dividendo, VA / VB = (2+1) / (2−1) = 3 / 1.
Given VA = 81 km/h ⇒ VB = 27 km/h. Total Distance (LA + LB) = Relative speed (Opposite) × Time = (81 + 27) × (5/18) × 36 = 1080 m.
LA = 1080 − 600 = 480 m.
∴ The correct answer is 480 m.
Alternate Method
Given: Speed of Train A (VA) = 81 km/h, Length of Train B (LB) = 600 m.
Formula: Distance = Relative Speed × Time; 1 km/h = 5/18 m/s.
⇒ Let LA be length of Train A and VB be speed of Train B.
⇒ Case 1 (Same direction): LA + 600 = (81 − VB) × (5/18) × 72
⇒ LA + 600 = (81 − VB) × 20 ... (i)
⇒ Case 2 (Opposite direction): LA + 600 = (81 + VB) × (5/18) × 36
⇒ LA + 600 = (81 + VB) × 10 ... (ii)
⇒ Equating (i) and (ii): (81 − VB) × 20 = (81 + VB) × 10
⇒ 162 − 2VB = 81 + VB ⇒ 3VB = 81 ⇒ VB = 27 km/h
⇒ LA + 600 = (81 + 27) × 10 = 1080
⇒ LA = 1080 − 600 = 480 m
∴ The correct answer is 480 m.
Additional Information
Relative Speed Concept
When two objects move in the same direction, relative speed is |V1 − V2|. When moving in the opposite direction, it is V1 + V2.
Crossing Distance
To fully cross or overtake each other, two trains must cover a distance equal to the sum of their lengths (L1 + L2).
Speed Conversion
To convert speed from km/h to m/s, multiply by 5/18. To convert from m/s to km/h, multiply by 18/5.