s=48km
t_1=4h
t_2=3h
v=? (prędkość własna)
v_p=? (prędkość rzeki)
v_1=v-v_p pod prąd
v_2=v+v_p z prądem
v=\frac{s}{t}
\left \{ {{v-v_p=\frac{48}{4}} \atop {v+v_p=\frac{48}{3}}} \right.
\left \{ {{v-v_p=12} \atop {v+v_p=16}} \right.
\left \{ {{v=12+v_p} \atop {12+v_p+v_p=16}} \right.
\left \{ {{v=12+v_p} \atop {12+2v_p=16}} \right.
\left \{ {{v=12+v_p} \atop {2v_p=16-12}} \right.
\left \{ {{v=12+v_p} \atop {2v_p=4}} \right.
\left \{ {{v=12+v_p} \atop {v_p=2}} \right.
\left \{ {{v=12+2} \atop {v_p=2}} \right.
\left \{ {{v=14} \atop {v_p=2}} \right.