Given:
Requirement of milk in gallons = 330
Let the gallons needed of milk containing 7% butterfat = x
Let the gallons needed of milk containing 1% butterfat = y
To fulfill the requirement,

And,

From equation (1),

Substitute in (2),

Solving further,

Therefore, the gallons required of milk containing 7 % butterfat are 275 and gallons required of milk containing 1% butterfat are 330-275= 55