[tex]\text{Notam cu x=pretul unei farfurie, y=pretul unei cani}\\
\left \{ {3x+9y=51|\cdot 3} \atop {9x+3y=57}} \right. \Leftrightarrow \left \{ {{9x+27y=153} \atop {9x+3y=57}} \right. \\
\text{Daca scadem relatiile:} /24y=96\Rightarrow \boxed{y=4\ lei}\\
3x+36=51\\
3x=15\Rightarrow \boxed{x=5\ lei}\\
R:\text{O farfurie costa 4 lei,iar o cana costa 5 lei.}[/tex]