A train 150 metres long crosses a milestone in 15 seconds and crosses another train of the same length travelling in the opposite direction in 12 seconds. The speed of the second train in km/hr is

Speed of first train $=frac{150}{15}=10 mathrm{~m} / mathrm{sec}$

Case-II,

Time $=frac{ ext { Length of both trains }}{ ext { Relative speed }}$

$12=frac{2 imes 150}{10+x}$

Where speed of second train = x m/sec

$Rightarrow 120+12 x=300$

$Rightarrow 12 x=300-120=180$

$Rightarrow x=frac{180}{12}=15 mathrm{~m} / mathrm{sec} .=frac{15 imes 18}{5}=54 mathrm{~km} / mathrm{h} .$