Solution:
Given the equations:
[tex]\begin{gathered} x+y=70\text{ ---- equation 1} \\ x-y=30\text{ ----- equation 2} \end{gathered}[/tex]
To solve for an expression equal to y, using the first equation, we make y the subject of the formula in equation 1.
Thus, from equation 1,
[tex]\begin{gathered} x+y=70 \\ \end{gathered}[/tex]
Add -x to both sides of the equation.
Thus,
[tex]\begin{gathered} -x+x+y=-x+70 \\ \Rightarrow y=-x+70 \end{gathered}[/tex]
Hence, we have
[tex]y=-x+70[/tex]
The third option is the correct answer.
