Question

x sobre y=12 sobre nove sendo que x+y=7

Answer 1

$\begin{cases}x+y=7\\ \dfrac{x}{y} = \dfrac{12}{9} \Rightarrow \dfrac{x+y}{y}= \dfrac{12+9}{9} \Rightarrow \dfrac{7}{y} = \dfrac{21}{9} \dfrac{:3}{:3} \Rightarrow \dfrac{7}{y} = \dfrac{7}{3}\Rightarrow y=3 \end{cases}\\\\x+y=7 \Rightarrow x + 3=7 \Rightarrow x=4$