Question

Train cross a bridge 300long in 15sec and another 450long in 21sec find speed and length

Answer 1

$\textrm{x = length of train}\\\textrm{v = speed of train = constans}\\t_1,\ t_2\textrm{ = times of crossing length of the bridges and the train x}\\\textrm{}\\v=\frac{s_1+x}{t_1}=\frac{300+x}{15}\\\\v=\frac{s_2+x}{t_2}=\frac{450+x}{21}\\\\\frac{300+x}{15}=\frac{450+x}{21}\\\\21x+21\cdot300=15x+15\cdot450\\6x=450\\x=75\ [m]\\\\v=\frac{300+75}{15}=25\ [m/s]=25\cdot\frac{3600}{1000}\ [km/h]=90\ [km/h]$