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

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
= [ 2x/((x / 18 ) + (x / 12)) ] = 2x / [(2x + 3x) / 36] = 2x * 36 / 5x
= 72 / 5
= 14.4 seconds.