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

114.4 seconds
215.5 seconds
318.8 seconds
420.2 seconds

Answer:- 1

Explanation:-

Solution:

Let length of each train be x meter.
Then, speed of 1^st train = ^x/₁₈ m/sec;
Speed of 2^nd train = ^x/₁₂ m/sec;

Now,
When both trains cross each other, time taken;

\begin{aligned} = [\frac{2 x}{{(\frac{x}{18}) + (\frac{x}{12})}}] \\ = \frac{2 x}{[\frac{(2 x + 3 x)}{36}]} \\ = \frac{2 x \times 36}{5 x} \\ = \frac{72}{5} \\ = 14.4 seconds \end{aligned}

$\eqalign{ & = \left[ {\frac{{2x}}{{\left\{ {\left( {\frac{x}{{18}}} \right) + \left( {\frac{x}{{12}}} \right)} \right\}}}} \right] \cr & = \frac{{2x}}{{\left[ {\frac{{\left( {2x + 3x} \right)}}{{36}}} \right]}} \cr & = \frac{{2x \times 36}}{{5x}} \cr & = \frac{{72}}{5} \cr & = 14.4\,{\text{seconds}} \cr}$