The first step is to define the system of equations.
Take x and y as the cost of a small box and a large box respectively.
[tex]\begin{gathered} 2x+y=12 \\ 3x+y=16 \end{gathered}[/tex]Solve the system.
[tex]\begin{gathered} 2x+y=12 \\ y=12-2x \\ 3x+y=16 \\ y=16-3x \end{gathered}[/tex][tex]\begin{gathered} 12-2x=16-3x \\ 3x-2x=16-12 \\ x=4 \end{gathered}[/tex][tex]\begin{gathered} y=12-2x \\ y=12-2\cdot4 \\ y=12-8 \\ y=4 \end{gathered}[/tex]A small box costs $4 and a large box costs $4 too.