[tex]Dane:\\s_1 = s\\v_1 = 60\frac{m}{h}\\s_2 = s\\v_2 = 40\frac{km}{h}\\Szukane:\\v_{sr} = ?\\\\Rozwiazanie\\\\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}{t} = \frac{2s}{\frac{s}{v_1}+\frac{s}{v_2}}=\frac{2}{\frac{1}{v_1}+\frac{1}{v_2}}=\frac{2}{\frac{v_1+v_2}{v_1v_2}}=\frac{2v_1v_2}{v_1+v_2}\\\\v_{sr} = \frac{2\cdot60\frac{km}{h}\cdot40\frac{km}{h}}{60\frac{km}{h}+40\frac{km}{h}}\\\\\boxed{v_{sr} = 48\frac{km}{h}}[/tex]
