Aprinter charges a fixed setup cost plus $0.40 for every 40 copies. if 320 copies cost $43.20 how much will it cost to print 1200 copies

So if we take this equation down a set by making 1200 into 120 and 40 into 4 so if we divide 120 by 4 then we get 30 so we times 30 by .4 and get 12 we then add the zero on the end and get $120