C. 500 m
Feedback Given, Length of the train = 800 m
Speed of the train = 78 km/hr (convert km/hr to m/sec)
= 78 × (5/18) m /sec
= 21.667 m /sec
=> Speed of the train = 21.667 m /sec
Time taken by the train to cross the tunnel = 1 min = 60 sec
To find the length of the tunnel:
Time taken by train to cross tunnel = (Length of tunnel + Length of train) / speed of the train
=> 60 = (Length of tunnel + 800) / 21.667
=> 60 × 21.667 = Length of tunnel + 800
=> 1300 = Length of tunnel + 800
=> 1300 – 800 = Length of tunnel
=> 500 = Length of tunnel
Thus, the Length of tunnel = 500 m