a) 45 km/hr
b) 50 km/hr
c) 54 km/hr
d) 55 km/hr
Answer: b
Explanation:
$$\eqalign{
& {\text{Speed}}\,{\text{of}}\,{\text{the}}\,{\text{train}}\,{\text{relative}}\,{\text{to}}\,{\text{man}} = {\frac{{125}}{{10}}} {\text{ m/sec}} \cr
& = {\frac{{25}}{2}} {\text{ m/sec}} \cr
& = {\frac{{25}}{2} \times \frac{{18}}{5}} {\text{ km/hr}} \cr
& = 45\,{\text{km/hr}} \cr
& {\text{Let}}\,{\text{the}}\,{\text{speed}}\,{\text{of}}\,{\text{the}}\,{\text{train}}\,{\text{be}}\,x\,{\text{km/hr}}. \cr
& \text{Then, relative speed} = \left( {x – 5} \right)\,{\text{km/hr}} \cr
& x – 5 = 45 \cr
& \Rightarrow x = 50\,{\text{km/hr}} \cr} $$
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