Answer:
20 quarts of 6% butterfat milk and 40 quarts of 3% butterfat milk is used to make 60 quarts of 4% butterfat milk.
Step-by-step explanation:
We are given the following in the question:
Let x quarts of 6% butterfat milk be used and y quarts of 3% butterfat milk be used to make 60 quarts of 4% butterfat milk.
Thus, we can write the equation:

Total amount of 3% butterfat milk = 60
Thus, we can write the equation:

Solving the two equation, we get,

Thus, 20 quarts of 6% butterfat milk and 40 quarts of 3% butterfat milk is used to make 60 quarts of 4% butterfat milk.