Explanation:

- To find the length of the train, let's call the length of the train L meters.

- When crossing a bridge, the train covers its own length plus the length of the bridge.

- Speed = Total distance / Time

- For the 600 m bridge:

Total distance = L + 600, Time = 80 s ?

Speed = (L + 600) / 80

- For the 300 m bridge:

Total distance = L + 300, Time = 60 s

Speed = (L + 300) / 60

- The speed is the same in both cases, so:

(L + 600) / 80 = (L + 300) / 60

Cross-multiplied:

60(L + 600) = 80(L + 300)

60L + 36000 = 80L + 24000

36000 - 24000 = 80L - 60L

12000 = 20L

L = 600 meters

- Reviewing options:

- Option 1: 120 m ?

- Option 2: 130 m ?

- Option 3: 600 m

- Option 4: 110 m ?

- Correct answer: Option 3 – 600 m