Two trains of equal length, running in opposite directions, pass a pole in 18 and 12 seconds. The trains will cross each other in:

Answer: Option A

Let length of each train be x meter.
Then, speed of 1^st train = $$\frac{x}{18}$$ m/sec
Speed of 2^nd train = $$\frac{x}{12}$$ m/sec
Now,
When both trains cross each other, time taken
$$\begin{array}{rl}& =\frac{2x}{\frac{x}{18}+\frac{x}{12}}\\ & =\frac{2x}{\frac{2x+3x}{36}}\\ & =\frac{2x\times 36}{5x}\\ & =\frac{72}{5}\\ & =14.4\text{seconds}\end{array}$$