Calculate speed of the train

When a train passes a pole, it covers its own length. Using $\text{Speed} = \frac{\text{Distance}}{\text{Time}}$, we find the speed.

$v = \frac{200 \text{ m}}{10 \text{ s}} = 20 \text{ m/s}$

Calculate total distance to cover

To cross the platform, the train must cover its own length plus the platform length.

$D_{total} = 200 \text{ m} + 300 \text{ m} = 500 \text{ m}$

Calculate time taken

Divide total distance by the constant speed of the train to find the time taken.

$t = \frac{500 \text{ m}}{20 \text{ m/s}} = 25 \text{ s}$