a) 10
b) 18
c) 36
d) 72
Answer: c
Explanation:
$$\eqalign{
& {\text{Let}}\,{\text{the}}\,{\text{speed}}\,{\text{of}}\,{\text{each}}\,{\text{train}}\,{\text{be}}\,x\,{\text{m/sec}}. \cr
& {\text{Then,}}\,{\text{relative}}\,{\text{speed}}\,{\text{of}}\,{\text{the}}\,{\text{two}}\,{\text{trains}} = 2x\,{\text{m/sec}} \cr
& {\text{So}},\,2x = \frac{{ {120 + 120} }}{{12}} \cr
& 2x = 20 \cr
& x = 10 \cr
& {\text{Speed}}\,{\text{of}}\,{\text{each}}\,{\text{train}} = 10\,{\text{m/sec}} \cr
& = {10 \times \frac{{18}}{5}} \,{\text{km/hr}} \cr
& = 36\,{\text{km/hr}} \cr} $$
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