[latex]Dane:\ V_{1}=60frac{km}{h}\ V_{2}=20frac{km}{h}\ Szukam:\ V_{sr}=?\ Rozwiazanie:\ ogolnie:\ V=frac{s}{t}=> t=frac{s}{V} \ \ Wiec:\ niech: s-droga z A do B/droga z B do A\ V_{sr}=frac{s+s}{t_{1}+t_{2}}\ ale:\ t_{1}=frac{s}{V_{1}} oraz: t_{2}=frac{s}{V_{2}}\ To:\ V_{sr}=frac{2s}{frac{s}{V_{1}}+frac{s}{V_{2}}}[/latex] [latex]V_{sr}=frac{2s}{s(frac{1}{V_{1}}+frac{1}{V_{2}}}\ \ V_{sr}=frac{2}{frac{1}{V_{1}}+frac{1}{V_{2}}}=frac{2}{frac{V_{2}+V_{1}}{V_{1}V_{2}}}=frac{2V_{1}V_{2}}{V_{1}+V_{2}}\ \ V_{sr}=frac{2*60*20}{60+20}\ \ V_{sr}=frac{2400}{80}=30frac{km}{h}[/latex]
