A train speed 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 / y = 15
=> y = x / 15
(x + 100) / 25 = x / 15
=> 15(x + 100) = 25x
=> 15x + 1500 = 25x
=> 1500 = 10x
=> x = 150 m.