Solve for a
a=\frac{21-3b}{2}
Solve for b
b=-\frac{2a}{3}+7
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80a=840-120b
Subtract 120b from both sides.
\frac{80a}{80}=\frac{840-120b}{80}
Divide both sides by 80.
a=\frac{840-120b}{80}
Dividing by 80 undoes the multiplication by 80.
a=\frac{21-3b}{2}
Divide 840-120b by 80.
120b=840-80a
Subtract 80a from both sides.
\frac{120b}{120}=\frac{840-80a}{120}
Divide both sides by 120.
b=\frac{840-80a}{120}
Dividing by 120 undoes the multiplication by 120.
b=-\frac{2a}{3}+7
Divide 840-80a by 120.
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