A train speeds past a pole in 15 seconds and a platform 100 m long in 25 seconds. Its length is:

Let the length of the train be x metres and its speed by y m/sec.

Then,	x	= 15          y =	x	.
	y		15

	x + 100	=	x
	25		15

 15(x + 100) = 25x

 15x + 1500 = 25x

 1500 = 10x

 x = 150 m.