A train moves past a telegraph post and a bridge 264 m long in 8 seconds and 20 seconds respectively. What is the speed of the train?

A. 79.2 km/hr
B. 70 km/hr
C. 64.7 km/hr
D. 80.1 km/hr

Solution:

 

\[\left. \begin{array} { l } { \text { Let the length of the train be } x and}\\
{\text { metres its speed by } y \text { m/sec. } } \\ { \text { Then, } \frac { x } { y } = 8 \Rightarrow x = 8 y } \\ { \text { Now, } \frac { x + 264 } { 20 } = y } \\ { \Rightarrow 8 y + 264 = 20 y } \\ { \Rightarrow y = 22 } \\ { \therefore \text { Speed } = 22 m / sec = ( 22 \times \frac { 18 } { 5 } ) km / hr }\\
{= 79.2 km / hr } \end{array} \right.\]

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