Two trains of equal lengths take 10 seconds and 15 seconds respectively to cross a telegraph post. If the length of each train be 120 metres, in what time (in seconds) will they cross each other travelling in opposite direction?

Given Data

• Equal train lengths = 120 m each
• Times to pass a post: 10 s and 15 s
• Required: time to cross each other in opposite directions

Step 1: Find individual speeds
v₁ = 120/10 = 12 m/sv₂ = 120/15 = 8 m/s

Step 2: Crossing each other
Relative speed (opposite) = 12 + 8 = 20 m/sDistance = 120 + 120 = 240 mt = 240 ÷ 20 = 12 s

Checks & Common Pitfalls

• Opposite directions ⇒ add speeds.
• Add lengths for complete crossing.

Final Answer
They will cross in 12 s.