How much is the labor fee and how much is each quart of oil?

Question

Two cars got an oil change at the same auto shop. The shop charges customers for each quart of oil plus a flat fee for labor. The oil change for one car required 5 quarts of oil and cost $19.80. The oil change for the other car required 7 quarts of oil and cost $22.30. How much is the labor fee and how much is each quart of oil?

Answer 1

5Q + L = 1980

7Q + L -2230 subtract 1st equation from the 2nd to get

2Q = 250

Q = 125 = $1.25 per quart of oil

L= 1980-5(125) = 1980-625 = 1355 = $13.55 for labor