Solve for y
y=\frac{81-x}{4}
Solve for x
x=81-4y
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4y=81-x
Subtract x from both sides.
\frac{4y}{4}=\frac{81-x}{4}
Divide both sides by 4.
y=\frac{81-x}{4}
Dividing by 4 undoes the multiplication by 4.
x=81-4y
Subtract 4y from both sides.
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