x - cena serka
y - cena jogurtu
[tex]\left \{ {{4x+3y=20-3,1} \atop {3x+5y=20-2,1}} \right. \\\left \{ {{4x+3y=16,9\ |*(-3)} \atop {3x+5y=17,9\ |*4}} \right. \\\left \{ {{-12x-9y=-50,7} \atop {12x+20y=71,6} \right|+\\\left \{ {{11y=20,9\ |:11} \atop {12x+20y=71,6} \right\\\left \{ {{y=1,9} \atop {12x+20*1,9=71,6} \right\\\left \{ {{y=1,9} \atop {12x+38=71,6} \right\\\left \{ {{y=1,9} \atop {12x=33,6\ |:12} \right\\\left \{ {{y=1,9} \atop {x=2,8} \right[/tex]
Odp: Serek kosztował 2,80 zł, a jogurt 1,90 zł.
