Question

Train a passes a lamp post in 9 seconds and 70 mlong platform in 30 seconds. how much time will the same train take to cross a platform which is 800m long

Answer 1

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{aligned}
=> \frac{x+100}{25} = \frac{x}{15} \\
x = 150 \text{ meters}
\end{aligned}

So length of the train is 150 meters