[tex] \left \{ {a+b=56} \atop {a=4+b}} \right. \ \textless \ =\ \textgreater \ \left \{ {{a=4+b} \atop {4+b+b=56}} \right. \ \textless \ =\ \textgreater \ \left \{ {{a=4+b} \atop {4+2b=56}} \right. \ \textless \ =\ \textgreater \ \left \{ {{a=4+b} \atop {2b=52 | :2}} \right. [/tex]<=>[tex] \left \{ {{a=4+b} \atop {b=26}} \right. =\ \textgreater \ a=4+26; a=30 km; b=26 km[/tex]
