Answer
$20.93$ gallons of cream and $29.07$ gallons of milk
Work Step by Step
Step 1. Assume we need $x$ gallons of cream and $y$ gallons of milk; the total amount is $x+y=50$ gallons which gives $x=50-y$
Step 2. The total amount of butterfat is given by $0.25x+0.035y=50(0.125)$ (volume times percentage)
Step 3. Substitute $x=50-y$ in the second equation; we have $0.25(50-y)+0.035y=50(0.125)$ which leads to $12.5-0.25y+0.035y=6.25$; thus $y\approx29.07$ gallon, and $x\approx20.93$ gallon.
Step 4. We conclude that we need $20.93$ gallons of cream and $29.07$ gallons of milk
