Respuesta :

[tex]\begin{gathered} 0.12y+0.04(y+8000)=1440 \\ 0.12y+0.04y+320=1440 \\ 0.16y=1440-320 \\ 0.16y=1120 \\ y=\frac{1120}{0.16} \\ y=7000 \end{gathered}[/tex]

The solution set does have only one element: {7000}