A train 900 m long is running at 108 km/h. How long will it take to clear a 900 m long platform completely?

The correct answer is 60 s.

When a train completely clears a platform, it must travel a total distance equal to the length of the train plus the length of the platform. This is because the train starts clearing the platform the moment its engine reaches the start of the platform, and it finishes only when the last coach has fully crossed the end of the platform.

Step 1: Identify the given values

Length of train = 900 m
Length of platform = 900 m
Speed of train = 108 km/h

Step 2: Convert speed from km/h to m/s

To work with metres and seconds consistently, we convert using the relation:

1\text{km/h}=\frac{5}{18}\text{m/s}

\text{Speed}=108\times \frac{5}{18}=30\text{m/s}

Step 3: Calculate the total distance to be covered

For the train to completely clear the platform, it must travel:

\text{Total Distance}=\text{Length of Train}+\text{Length of Platform}

\text{Total Distance}=900+900=1800\text{m}

Step 4: Apply the Time-Distance formula

\text{Time}=\frac{\text{Distance}}{\text{Speed}}=\frac{1800}{30}=60\text{seconds}

Conclusion: The train will take 60 seconds to completely clear the platform.