For the fast food company: Cost of 100 hamburger buns = $120 Hence, cost of 5 hamburger buns = (5*120)/100 = $6 For the customer: Cost of 3 hamburger buns = $6 Hence, cost of 100 hamburger buns = (100*6)/3 = $200 The fast food company bought 100 buns for $120, and sold those 100 buns for $200. Hence, the profit per 100 buns = 200 - 120 = $80