So I need help with finding government expenditure on the subsidy scheme in this market, I know it is size of the subsidy x quantity which is $4 x 400 = $1600, but our teacher gave us a question that why do we multiply it by 400 and why not by the difference between 400 and 300 (so 100). I know it's 400 but I don't know the reasoning behind it, could anyone explain why?