Chapter 5, Systems of Equations and Inequalities - Section 5.1 - Systems of Linear Equations in Two Variations - 5.1 Exercises - Page 447: 63

$200$ gallons of normal and $80$ gallons of premium gas was sold.

Let $x$ be the gallon of normal gas sold. Then the gallon of premium gas sold is $280-x$. Then our equation is: $2.2x+3(280-x)=680\\2.2x+840-3x=680\\0.8x=160\\x=200$ Thus $200$ gallons of normal and $280-200=80$ gallons of premium gas was sold.