A train 800 metres long is running at a speed of 78 km/hr. If it crosses a tunnel in 1 minute, then the length of the tunnel (in meters) is:

Given Data

• Train length = 800 m
• Speed = 78 km/hr
• Time to cross tunnel = 1 minute = 60 s
• Required: tunnel length

Step 1: Convert speed to m/s
78 × (5/18) = 21.6667 m/s

Step 2: Total distance to clear the tunnel
Distance = speed × time = 21.6667 × 60 = 1300 m1300 = train length + tunnel length = 800 + LL = 1300 − 800 = 500 m

Checks & Common Pitfalls

• Add the train length to the tunnel length for full clearance.
• Mind the unit conversion before multiplying by time.

Final Answer
The tunnel length is 500 m.