[tex]Dane:\\s = 120 \ km\\v_1 = 60\frac{km}{h}\\v_2 = 6\frac{km}{h}\\Szukane:\\v_{sr} = ?\\\\v_{sr} = \frac{s_{c}}{t_{c}}\\\\s_{c} = 2s\\t_{c} = t_1+t_2 = \frac{s}{v_1} + \frac{s}{v_2}\\\\v_{sr} = \frac{2s}{\frac{s}{v_1}+\frac{s}{v_2}} = \frac{2}{\frac{1}{v_1}+\frac{1}{v_2}} = \frac{2v_1v_2}{v_1+v_2}\\\\\\v_{sr} = \frac{2\cdot60\frac{km}{h}\cdot6\frac{km}{h}}{60\frac{km}{h}+6\frac{km}{h}}=\frac{720\frac{km}{h}}{66}[/tex]
[tex]\boxed{v_{sr} = 10,(90)\frac{km}{h}\approx10,91\frac{km}{h}}[/tex]
