A) 69.5 km/hr
B) 70 km/hr
C) 79 km/hr
D) 79.2 km/hr
Correct Answer: D
Solution :
Let the length of train be \[x\] metres. Then, speed of train when it passes a telegraph post\[=\frac{x}{8}\text{m/sec}\] and speed of train, when it passes the bridge\[=\frac{x+264}{20}\] Clearly, \[\frac{x}{8}=\frac{x+264}{20}\] \[\Rightarrow \] \[\frac{x}{2}=\frac{x+264}{5}\] \[\Rightarrow \] \[5x=2x+528\] \[\Rightarrow \] \[3x=528\] \[\Rightarrow \] \[x=\frac{528}{3}\] \[\therefore \]Speed of train \[=\frac{176}{8}=22\,\,\text{m/sec}\] \[=22\times \frac{18}{5}\text{Kmph}\] \[=79.2\,\,\text{kmph}\]
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