[tex]d=d_1+d_2[/tex]
[tex]d_1=d_2=\frac{d}{2}[/tex]
[tex]t1=\frac{d_1}{v_1}=\frac{\frac{d}{2}}{v_1}=\frac{d}{2v_1}[/tex]
[tex]t_2=\frac{d_2}{v_2}=\frac{\frac{d}{2}}{v_2}=\frac{d}{2v_2}[/tex]
[tex]t=t_1+t_2[/tex]
[tex]v_{med}=\frac{d}{t}[/tex]
[tex]v_{med}=\frac{d}{\frac{d}{2v_1}+\frac{d}{2v_2}}[/tex]
[tex]v_{med}=\frac{d}{\frac{d(v_1+v_2}{2v_1v_2}}[/tex]
[tex]v_{med}=\frac{2dv_1v_2}{d(v_1+v_2)}[/tex]
[tex]v_{med}=\frac{2v_1v_2}{v_1+v_2}[/tex]
[tex]54\frac{km}{h}=15\frac{m}{s}[/tex]
[tex]v_{med}={2*15*20}{15+20}=\frac{600}{35}=17,14\frac{m}{s}[/tex]
