Answer:
I believe that the demand equation is incomplete, since there is no price (P).
If we just solve this equation like it is, the quantity demanded = 80 - 12 = 68 gallons
if we add the price into the equation:
Q = 80 - 12P
since this is a competitive market, in order to maximize profits, marginal revenue = marginal cost = $4.10 per gallon (not $410).
That means that both companies will sell gasoline at $4.10 per gallon
equilibrium quantity = 80 - (12 x 4.10) = 30.8 gallons of gasoline