Help with this word problem, I'm confused.

Question

Find how many quarts of 5% butterfat milk and 1% butterfat milk should be mixed to yield 120 quarts of 2% butterfat milk.

Answer 1

If x is the number of quarts of 5% and y is the number of quarts of 1% milk, then x+y=120 (1) to end up with 120 quarts. 

To produce 2% buttermilk, you'd use the equation 5x+1y=2*120. (2)

You can rearrange (1) to produce the equation y=120-x (3) and subsitute into (2) to form the equation 5x+120-x=240 (4). This simplifies to 4x=120 (5) which is solved by x=30 (6). When you substitute this value back into (1), you find y=90 (7), meaning 30 quarts of 5% and 90 quarts of 1% would be used to create this mixture.