a) 10
b) 12
c) 15
d) 20
Answer: b
Explanation:
$$\eqalign{
& {\text{Speed}}\,{\text{of}}\,{\text{the}}\,{\text{first}}\,{\text{train}} \cr
& = {\frac{{120}}{{10}}} \,{\text{m/sec}} \cr
& = 12\,{\text{m/sec}} \cr
& {\text{Speed}}\,{\text{of}}\,{\text{the}}\,{\text{second}}\,{\text{train}} \cr
& {\frac{{120}}{{15}}} \,{\text{m/sec}} \cr
& = 8\,{\text{m/sec}} \cr
& {\text{Relative}}\,{\text{speed}} = {12 + 8} = 20\,{\text{m/sec}} \cr
& {\text{Required}}\,{\text{time}} \cr
& = {\frac{{ {120 + 120} }}{{20}}} \,{\text{ sec}} \cr
& = 12\,{\text{sec}} \cr} $$
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