a) 15.5 seconds
b) 14.4 seconds
c) 20.2 seconds
d) 18.8 seconds
Answer: b
Explanation: Let length of each train be x meter.
Then, speed of 1st train = $$\frac{x}{{18}}$$ m/sec
Speed of 2nd train = $$\frac{x}{{12}}$$ m/sec
When both trains cross each other, time taken
$$\eqalign{
& = {\frac{{2x}}{{ { {\frac{x}{{18}}} + {\frac{x}{{12}}} } }}} \cr
& = \frac{{2x}}{{ {\frac{{ {2x + 3x} }}{{36}}} }} \cr
& = \frac{{2x \times 36}}{{5x}} \cr
& = \frac{{72}}{5} \cr
& = 14.4\,\,{\text{seconds}} \cr} $$
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