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 \[=78\,\text{km/h}\] \[=\frac{78\times 1000}{60\times 60}m/s\] \[=\frac{65}{3}m/s\] \[\therefore \] \[\text{Speed=}\frac{\text{Distance}}{\text{Time}}\] \[\Rightarrow \] \[\frac{65}{3}=\frac{800+x}{60}\] \[\Rightarrow \] \[(800+x)\times 3=65\times 60\] \[\Rightarrow \] \[800+x=65\times 20\] \[\Rightarrow \] \[800+x=65\times 20\] \[\Rightarrow \] \[x=1300-800=500\] \[\therefore \] Length of tunnel \[=500\,m\]
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