[tex]dane:\\s_1 = 15 \ km\\t_1 = 30 \ min = \frac{1}{2} \ h\\s_2 = 20 \ km\\v_2 = 30\frac{km}{h}\\s_3 = 5 \ km\\v_3 = 15\frac{km}{h}\\szukane:\\v_{sr} = ?\\\\Rozwiazanie\\\\v_{sr} = \frac{s_{c}}{t_{c}}[/tex]
[tex]s_{c} = s_1 + s_2 + s_3 = 15 \ km + 20 \ km + 5 \ km = 40 \ km\\\\t_{c} = t_1 + t_2 + t_3\\\\t_1 = \frac{1}{2} \ h\\\\t_2 = \frac{s_2}{v_2} = \frac{20 \ km}{30\frac{km}{h}} = \frac{2}{3} \ h\\\\t_3 = \frac{s_3}{v_3} = \frac{5 \ km}{15\frac{km}{h}} = \frac{1}{3} \ h[/tex]
[tex]t_{c} = \frac{1}{2} \ h + \frac{2}{3} \ h + \frac{1}{3} \ h =\frac{3+4+2}{6} \ h = \frac{9}{6} \ h = 1,5 \ h[/tex]
[tex]v_{sr} = \frac{40 \ km}{1,5 \ h}=26,(6)\frac{km}{h}\\\\\underline{v_{sr} =26,67\frac{km}{h}}[/tex]