Fan and ! . the cost of coffee is determined by the type of coffee beans that are mixed together. the cost of a local mix of arabica and robusta beans is modeled by the equation below where a = the pounds of arabica and r = pounds of robusta.. 700a + 1200r = 1,000,000. if a mixture contains 1000 pounds of arabica beans, how many pounds of robusta beans should be in the mixture? . r = pounds