[tex]Dane:\\v_1 = 10\frac{km}{h}\\t_1 = 1,5 \ h\\s_2 = 20 \ km\\v_2 = 15\frac{km}{h}\\Szukane:\\v_{sr} = ?\\\\Rozwiazanie\\\\v_{sr} = \frac{s_{c}}{t_{c}}\\\\s_{c} = s_1+s_2=v_1\cdot t_1 + s_2 = 10\frac{km}{h}\cdot1,5 \ h = 20 \ km = 15 \ km + 20 \ km = 35 \ km\\\\t_{c} = t_1+t_2 = t_1 + \frac{s_2}{v_2}=1,5 \ h + \frac{20 \ km}{15\frac{km}{h}} = 1,5 \ h + 1,33 \ h = 2,83 \ h\\\\\\v_{sr} = \frac{35 \ km}{2,83 \ h}\\\\\boxed{v_{sr}\approx12\frac{km}{h}}[/tex]
