v_1=60\frac{km}{h}
v_2=80\frac{km}{h}
v_{sr}=\frac{s+s}{t_1+t_2}=\frac{2s}{\frac{s}{v_1}+\frac{s}{_v_2}}=\frac{2s}{s(\frac{1}{v_1}+\frac{1}{v_2})}=\frac{2}{\frac{v_2+v_1}{v_1*v_2}}
v_{sr}=\frac{2v_1*v_2}{v_1+v_2}=\frac{2*60*80}{60+80}=\frac{9600}{140}=\frac{480}{7}\frac{km}{h} <-- odpowiedź
=68\frac{4}{7}\frac{km}{h}