Question 58629
Let x=amt of milk (gal) to be mixed
Then 35-x=amt of whipping cream (gal) to be mixed
We know that the amt of butterfat in the milk (x)(.02) plus the amt of butterfat in the whipping cream (35-x)(.09) equals the amt of butterfat in the final mixture (35)(.04). Thus, our equation to solve is:

(x)(.02)+(35-x)(.09)=(35)(.04) simplifying, we get
.02x+3.15-.09x=1.4 further simplifying, we have
-.07x=-1.75
x=25 gal
We need 25 gal of milk and 35-25=10 gal of whipping cream 
Hope this helps ----ptaylor