Explanation:

- The train speed is constant. It is the same whether it crosses a 1200-meter or 1600-meter bridge.

- Formula to find speed: Speed = Distance / Time.

- For the 1200-meter bridge:

- Distance = Length of the train + 1200 meters

- Calculated time = 16 seconds.

- For the 1600-meter bridge:

- Distance = Length of the train + 1600 meters

- Calculated time = 20 seconds.

- Use the two equations to form relationships:

- 16S = L + 1200

- 20S = L + 1600

- Solving these gives: L = 480 meters and Speed (S) = 90 m/s = 324 km/h.

- Correct speed option closest to 324 km/h:

- Option 1: 360 km/h

- Option 2: 450 km/h

- Option 3: 540 km/h

- Option 4: 270 km/h

Therefore, given options lack the correct speed calculation based on provided distance-time relations.