Answer: Quantity of Dark Chocolate used = 15 lb
Quantity of Milk Chocolate used = 35 lb
Explanation:
Let x = Quantity of Dark Chocolate.
y = Quantity of Milk Chocolate.
As per given , we have the following system of equations:

Multiply equation (1) by 2 , we get

Eliminate equation (3) from equation (2) , we get

Put x= 15 in (1) , we get

⇒ y=35
Therefore , Quantity of Dark Chocolate used = 15 lb
Quantity of Milk Chocolate used = 35 lb