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

Explanation:

Let the length of the train be x metres and its speed be 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.

Hence, option 3 is the correct answer.