x - wojsko Eryka
y - wojsko Roderyka
\left \{ {{30\%x+60\%y=40\%*3000} \atop {x+y=3000}} \right.
\left \{ {{0,3x+0,6y=0,4*3000} \atop {x=3000-y}} \right.
\left \{ {{0,3(3000-y)+0,6y=1200} \atop {x=3000-y}} \right.
\left \{ {{900-0,3y+0,6y=1200} \atop {x=3000-y}} \right.
\left \{ {{900+0,3y=1200} \atop {x=3000-y}} \right.
\left \{ {{0,3y=1200-900} \atop {x=3000-y}} \right.
\left \{ {{0,3y=300} \atop {x=3000-y}} \right.
\left \{ {{y=1000} \atop {x=3000-1000}} \right.
\left \{ {{y=1000} \atop {x=2000}} \right.
Eryk miał 2000 rycerzy, a Roderyk 1000.