sys of equations: lbs of coffee?

Alan asked: Forgot how to do these… step by step please.
A shop specializes in preparing blends of gourmet coffees. From Colombian, Costa Rican, and Kenyan coffees, the owner wishes to prepare 3-pounds bags that will sell for $8.50. The cost per pound of these coffees is $10, $6, and $8, respectively. The amount of [...]

How would you do this derivative problem?

nietzsche asked: Does anyone know how to do these problems? If you could show how to do it and what the answers would be, it would be greatly appreciated!! Thanks!
The quantity (in pounds) of a gourmet ground coffee that is sold by a coffee company at a price of p dollars per pound is [...]