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?

We are given:

• Time taken by Train A to cross a post = 10 seconds
• Time taken by Train B to cross a post = 15 seconds
• Length of each train = 120 meters

Step 1: Find speed of each train

Speed = Distance / Time
Speed of Train A = 120 / 10 = 12 m/s
Speed of Train B = 120 / 15 = 8 m/s

Step 2: Find relative speed when moving in opposite directions

Relative Speed = 12 + 8 = 20 m/s
Combined length = 120 + 120 = 240 meters

Step 3: Calculate time to cross each other

Time = Distance / Speed
= 240 / 20 = 12 seconds

Final Answer:

They will cross each other in 12 seconds