A train crosses a platform in 30 seconds and a pole in 18 sec. If speed of train is 45 km/hr, then find the length of 42. platform. 

Speed of train

$$\begin{gathered} =45\times \frac{5}{18} \\ =\frac{25}{2} \\ =12.5\mathrm{m}/\sec \end{gathered}$$

Length of train $= 12.5 \mathrm{x} 18 = 225 \mathrm{m}$

Let length of platform is $\mathrm{x} \mathrm{m}$. 

According to question,

$\begin{array}{l}\frac{225+x}{12.5}=30\\ \Rightarrow 225+x=12.5\times 30=375\\ \Rightarrow x=375-225=150\end{array}$

Hence, length of platform is $150 \mathrm{m}$.