[tex]d=d_1+d_2\\
d_1=d_2=\frac{d}{2}\\
t=t_1+t_2\\
t_1=\frac{d_1}{v_1}=\frac{d}{2v_1}\\
t_2=\frac{d_2}{v_2} =\frac{d}{2v_2}\\
d=v_m*t=>v_m=\frac{d}{\frac{d}{2v_1}+ \frac{d}{2v_2} }=\frac{1}{\frac{v_2+v_1}{2v_1v_2} }=\frac{2v_1v_2}{v_1+v_2}=\frac{2*30*15}{30+15}=\frac{900}{45}=20 km/h\frac{x}{y} [/tex]
