A gas station sells regular gas for $2.20 per gallon and premium gas for $3.00 a gallon. at the end of a business day 300 gallons of gas has been sold, and receipts totaled $700. how many gallons of each type of gas had been sold

You can use a system of equations to answer this question.

1st:  2.20r + 3p = 700 is showing how to get the total cost.
2nd: r + p = 300 shows how many gallons were sold.

If we solve r + p = 300 for r, we can then solve the first equation in terms of r only.

r + p =300
p = 300 - r  Use this in place of p in the first equation.

2.20r + 3p = 700
2.2r + 3(300 - r) = 700
2.2r + 900 - 3r = 700
       -900            -900
2.2r - 3r = -200
-0.8r = -200
-0.8     -0.8
r = 250 gallons

There were 250 regular gallons sold and 50 premium gallons sold (300 - 250).