Explanation:

To find the speed of the longer train:

- Total length to be crossed: 400 m (long train) + 300 m (short train) = 700 m.

- Time taken: 15 seconds.

- Relative speed: The combined speed of both trains in opposite directions.

Convert the speed of the shorter train to meters per second:

- 60 km/h = 60 * 5 / 18 m/s = 16.67 m/s.

Using the formula Relative Speed = Total Distance/Time, we get:

- 700/15 = 46.67 m/s.

The speed of the longer train:

- 46.67 - 16.67 = 30 m/s.

Convert to km/h:

- 30 * 18 / 5 = 108 km/h

- Correct Answer: 108