Answer: They should buy 12 pounds of City Roast and 8 pounds of French Roast.
Step-by-step explanation:
Let x be the weight of City Roast colombian coffee.
Let y be the weight of French Roast Colombian coffee.
Weight of blend = 20 pounds
So, our equation becomes

Cost of per pound of City Roast = $7.80
Cost of per pound of French Roast = $8.10
Cost of blend = $7.92
So, our equation becomes,

From Eq(1), we get that

Put this in Eq(2), we get that


Hence, they should buy 12 pounds of City Roast and 8 pounds of French Roast.