length of train which has passed through tunnel A train is travelling at a speed of 96 km/hr. It takes 3 s to enter a tunnel and 30 s more to pass through it completely. What is the length of the train?
HINT:

Entering the tunnel means crossing own length

Crossing the tunnel mean travelling own length + that of the tunnel
96 Km/hr equivalent to (80/3) m/s.
It takes 3s for a train to enter into the tunnel.

Hence, length of the train = (80/3) *3

Therefore, 80m is the length. 

The second part stating 30s is not really required for calculating the length.