Răspuns :
[tex] \left \{ {{2a+4b=68|*1=\ \textgreater \ 2a+4b=68} \atop {a+b=20|*(-2)=\ \textgreater \ -2a-2b=-40} \right.\\
2b=28=\ \textgreater \ b=28:2=\ \textgreater \ b=14\\
a+14=20=\ \textgreater \ a=20-14=\ \textgreater \ a=6\\
Proba:14+6=20\\
2*6+4*14=68\\
12+56=68\\
68=68
[/tex]