Let the length of the first train be x metres.

Then, the length of the second train is		x		metres.
		2

Relative speed = (48 + 42) kmph =		90 x	5		m/sec = 25 m/sec.
			18

	[x + (x/2)]	= 12 or	3x	= 300     or     x = 200.
	25		2

 Length of first train = 200 m.

Let the length of platform be y metres.

Speed of the first train =		48 x	5		m/sec =	40	m/sec.
			18			3

(200 + y) x	3	= 45
	40

 600 + 3y = 1800

 y = 400 m.