A) 77200
B) 500
C) 1300
D) 13
Correct Answer: B
Solution :
| When a train crosses a tunnel, it covers a distance equal to the sum of its own length and tunnel. |
| Let the length of tunnel be x m, |
| Speed = 78 km/h \[=\frac{78\times 1000}{60\times 60}\text{m/s}=\frac{65}{3}\text{m/s}\] |
| \[\therefore \] \[\text{Speed}=\frac{\text{Distance}}{\text{Time}}\] |
| \[\Rightarrow \] \[\frac{65}{3}=\frac{800+x}{60}\] |
| \[\Rightarrow \] \[(800+x)\times 3=65\times 60\] |
| \[\Rightarrow \] \[800+x=65\times 20\] |
| \[\Rightarrow \] \[x=1300-800=500\] |
| \[\therefore \] Length of tunnel = 500 m |
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