80. 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:

✅ The correct answer is option C.
Speed =

78 x
5

m/sec
=

65

m/sec.

18
3

Time = 1 minute = 60 seconds.
Let the length of the tunnel be x metres.

Then,

800 + x

=
65

60
3

3(800 + x) = 3900
x = 500.