A train, 800 meter long is running with a speed of 78 km/hr. It crosses a tunnel in 1 minute. What is the length of the tunnel ?

Let length of tunnel is x meter
Distance = 800+x meter
Time = 1 minute = 60 seconds
Speed = 78 km/hr = 78*5/18 m/s = 65/3 m/s
Distance = Speed*Time
$$\begin{gathered} \Rightarrow 800+x=\frac{65}{3}\times 60 \\ \Rightarrow 800+x=20\times 65=1300 \\ \Rightarrow x=1300-800=500 \end{gathered}$$
So the length of the tunnel is 500 meters.