A train A passes a fixed pole in 24 seconds. The same train completely crosses another train B of length 150 m coming from the opposite direction in 20 seconds. Train B’s speed is 54 km/h. Find the length and the speed of train A.
- AOption (a)
600 m and 135 km/h
- BOption (b)
500 m and 135 km/h
- COption (c)
900 m and 135 km/h
- DOption (d)
900 m and 134 km/h
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Correct answer
900 m and 135 km/h
Why this answer is correct
Shortcut Trick
Let the speed of Train A be SA. Length LA = SA × 24.
Speed of Train B = 54 × (5/18) = 15 m/s.
When crossing Train B in opposite direction: (LA + 150) = (SA + 15) × 20.
Substitute LA: 24SA + 150 = 20SA + 300 ⇒ 4SA = 150.
SA = 37.5 m/s = 37.5 × (18/5) = 135 km/h.
LA = 37.5 × 24 = 900 m.
∴ The correct answer is 900 m and 135 km/h.
Alternate Method
Given:
Time to pass fixed pole = 24 sec
Time to cross Train B = 20 sec
Length of Train B = 150 m
Speed of Train B = 54 km/h
Formula Used:
Distance = Speed × Time
Relative Speed (Opposite) = S1 + S2
Calculations:
⇒ Let Speed of Train A = v m/s and Length of Train A = L m.
⇒ From first condition: L = v × 24 → (i)
⇒ Speed of Train B in m/s = 54 × (5/18) = 15 m/s.
⇒ From second condition (crossing in opposite direction):
⇒ Total Distance = L + 150
⇒ Relative Speed = v + 15
⇒ (L + 150) = (v + 15) × 20 → (ii)
⇒ Substituting (i) in (ii): 24v + 150 = 20v + 300
⇒ 4v = 150 ⇒ v = 37.5 m/s
⇒ Speed of Train A in km/h = 37.5 × (18/5) = 135 km/h.
⇒ Length of Train A = 37.5 × 24 = 900 m.
∴ The correct answer is 900 m and 135 km/h.
Additional Information
Relative Speed Concept
For objects moving in the same direction, Relative Speed = S1 − S2. For opposite directions, Relative Speed = S1 + S2.
Unit Conversion
To convert km/h to m/s, multiply by 5/18. To convert m/s to km/h, multiply by 18/5.
Train Crossing a Pole vs Train
When a train crosses a pole/man, distance = length of train. When it crosses another train/platform, distance = sum of both lengths.
