A train speeds past a pole in 15 seconds and a platform 100 meter long in 25 seconds. What is length of the train ?

Let the length of the train is x meter and Speed of the train is y meter/second
Then x/y = 15 [because distance/speed = time]
=> y = 15/x
$$\begin{gathered} \Rightarrow \frac{x+100}{25}=\frac{x}{15} \\ x=150 meters \end{gathered}$$
So length of the train is 150 meters