Correct Answer - Option 3 : 500
Given:
Length of train = 800 m
Speed of train = 78 km/h
Time to cross the tunnel = 1 minute = 60 seconds
Concept used:
1 km/h = 5/18 m/s
Formula used:
Speed = (Length of train + Length of tunnel)/Time
Calculations:
Speed = (Length of train + Length of tunnel)/Time
⇒ 78 × (5/18) = (800 + x)/60
⇒ 65/3 = (800 + x)/60
⇒ 65 × 20 = 800 + x
⇒ 1300 = 800 + x
⇒ x = 500 m
∴ The length of the tunnel is 500 m.