Explanation:

Let’s break it down step by step:

- The first train moves at 72 km/hr. Convert to m/s: 72 × 1000 ÷ 3600 = 20 m/s.

- It crosses a post in 20 seconds. So, length of the first train = 20 × 20 = 400 m.

- Now, there’s another train going in the same direction at 54 km/hr. That’s 54 × 1000 ÷ 3600 = 15 m/s.

- The two trains cross each other in 1 minute 45 seconds = 105 seconds.

- Relative speed = 20 - 15 = 5 m/s, since they go the same way.

- Total distance covered while crossing = relative speed × total time = 5 × 105 = 525 m.

- This 525 m is the sum of the lengths of both trains.

- We know the first train is 400 m, so the second train is 525 - 400 = 125 m.

- The difference in length = 400 - 125 = 275 m.

Here’s what that means for the options:

- Option 1: 275 m

- Option 2: 225 m

- Option 3: 125 m

- Option 4: 175 m

- Option 5: 200 m

The correct answer is Option 1: 275 m.