[tex]x- amount\ of\ chicken-\ cost\ 6\$\ per\ pound\\
y-amount\ of sausage\ - cost\ 3\$\ per\ pound\\
z-amount\ of rice-\ cost\ 1\$\ per\ pound \\\\
x+y+z=13,5\ and\\
6*x+3*y+1z=42 \ and\\
z=2y\\\\
First\ substitude\ z=2y\ to \ both\ equations:\\
\left \{ {{x+y+2y=13,5} \atop {6x+3y+2y=42}} \right. \\\\
\left \{ {{x+3y=13,5} \atop {6x+5y=42}} \right. |First\ equation\ multiply\ by\ -6\\\\
\left \{ {{-6x-18y=-81 } \atop {6x+5y=42}} \right.|Adding\ equations\\
\+\ -\ -\ -\ -\ -\ -\- \\
-13y=-39|:-13[/tex][tex]y=3\\
x+3y=13,5\\
x+3*3=13,5\\
x=13,5-9=4,5\\
x=4,5\\
z=2y\\
z=2*3=6\\
She\ will \ use \ 4,5\ pounds\ of\ chicken, \ 3\ pounds\ of\ sausage\\
and\ 6\ pounds\ of \ rice
[/tex]