Question

x + y = 20
3X+ 4y = 72​

Answer 1

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$\begin{cases}\sf x+y=20\\\sf 3x+4y=72\end{cases}\\\sf\underline{multiplicando~a~1^{\underline a}~equac_{\!\!,}\tilde ao~por~-3~para~cancelar~x~temos:}\\+\underline{\begin{cases}\sf-\diagup\!\!\!\!\!3x-3y=-60\\\sf\diagup\!\!\!\!\!3x+4y=72\end{cases}}\\\sf y=12\checkmark\\\sf x+y=20\\\sf x+12=20\\\sf x=20-12\\\sf x=8\checkmark\\\huge\boxed{\boxed{\boxed{\boxed{\sf S=\{8,12\}}}}}$