A train 850 m long is running at a speed of 70 km/h. If it crosses a tunnel in $1\frac{1}{2}$ minute, then the length of the tunnel (in m) is

900

Speed of train =  70 km/h

= 70 × \(\frac{5}{18}\) m/sec

Total time taken to cross the tunnel = 1.5 minute = 90 sec

Total distance to travel = Length of train + Tunnel length

= 850 + Tunnel length 

Distance = Speed × Time

850 + Tunnel length = 70 × \(\frac{5}{18}\) × 90

850 + Tunnel length = 1750

Tunnel length = 900

The correct answer is option (3) : 900