500 m
Detail
Let the tunnel be x meters long
To Covert Km/h to m/s we use formual: [ s x (a/18) ]
So the speed of train in m/s =78km/h=78×5/18= 65/3 m/s
Time taken: 1 min=60 sec
Distance covered would be= x+800
because to cross the tunnel, the train must cover the distance equal to the sum of its length and tunnel’s length:
Now applying Speed=Distance/Time:
65/3 = x+800/60
x+800=65×20
x=1300–800
x=500 m