Answer :
If the gourmet shopkeeper mix 15 pounds of coffee beans which costs 3.0$ per pound with 10 pounds of coffee beans costing 4.25$ per pound.
The mixture of 25 pounds will cost 3.5$ per pound.
It can be found using
3*x + 4.25*(25 -x) = 25*3.50
The mixture of 25 pounds will cost 3.5$ per pound.
It can be found using
3*x + 4.25*(25 -x) = 25*3.50