A person travels along a straight road for half of the distance with a velocity 30km/h and the remaining half distance with a velocity 50km/h. Then the average velocity is given by

Given that    $s_1=\frac{s}{2}{s}_2=\frac{S}{2}$
                     $v_1=30km/h$
                     $v_2=50km/h$
$\therefore$ Time taken to cover first half of the distance  $t_1=\frac{s}{2v_1}$
The time taken to cover second half of the distance  $t_2=\frac{s}{2v_2}$
Average velocity  $\left(\overline{V}\right)=\frac{Totaldispalcement}{Totaltime}$

                                  $\begin{array}{l}=\frac{S_1+s_2}{t_1+t_2}\\ =\frac{\frac{s}{2}+\frac{s}{2}}{\frac{s}{2v_1}+\frac{s}{2v_2}}=\frac{2v_1{v}_2}{v_1+v_2}\\ =\frac{2\times 30\times 50}{30+50}\\ =\frac{2\times 30\times 50}{80}\\ =\frac{300}{8}=\frac{150}{4}=37.5\mathrm{km}/\mathrm{h}\end{array}$