Answer
That day, 200 gallons of regular gas and 80 gallons of premium gas were sold.
Work Step by Step
Let's assume that $x$ gallons of regular gas and $y$ gallons of premium gas had been sold that day. As $280$ gallons of gas was sold that day, Therefore-
$x+y$ = $280$ __eq.1
Now price for one gallon of regular gas is $ 2.20$ dollars and price for one gallon of premium gas is $ 3.00$ dollars, and total collection of the day was $680$ dollars. Therefore-
$2.20 \times x +3 \times y$ = $680$
i.e. $2.2 x +3 y$ = $680$ __eq.2
Substituting for $x$ in eq.2 from eq.1-
$2.2 (280-y) +3 y$ = $680$
i.e. $616-2.2y + 3 y$ = $680$
i.e. $616+ 0.8 y$ = $680$
i.e. $0.8 y$ = $680-616$ = $64$
i.e. $y$ = $\frac{64}{0.8}$ = $80$
Substituting for $y$ in eq.1-
$x+80$ = $280$
i.e. $x$ = $280-80$ = $200$
Thus 200 gallons of regular gas and 80 gallons of premium gas were sold that day.
