text: P = total cost\np = price per quart\nq = # of quarts\nc = constant price\n\n\n3.09 = 4p + c\nc = 3.09 - 4p\n\n1.65 = 2p + c\n1.65 = 2p + (3.09 - 4p)\n1.65 - 3.09 = 2p - 4p\n-1.44 = -2p\np = .72\n\n3.09 = 4(.72) + c\nc = 3.09 - 2.88\nc = .21\n\nP = .72q + .21\n\nWhat you are forgetting is that there can be a base constant price to which the variable price is added.  That constant being c.