SOLUTION: Trying to help my student and I cannot remember after 30+ years. Will you please explain each step in detail. I cannot even remember how to make an equation for this. Thank you!

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Question 16295: Trying to help my student and I cannot remember after 30+ years. Will you please explain each step in detail. I cannot even remember how to make an equation for this. Thank you!
How much whipping cream (9% butterfat) and how much 2% milk should be mixed to obtain 35 gallons of milk with 4% butterfat?
Answer by rapaljer(4671) About Me  (Show Source):
You can put this solution on YOUR website!
Let x = amount of whipping cream at 9% (.09). The rest of the 35 gallons is 2%, so
35-x = amount of milk at 2%.

The result is 35 gallons of 4% milk.
.09x + .02(35-x) = .04(35)
.09x + .70 - .02x = 1.40
.07x + .70 = 1.40
.07x + .70 -.70 = 1.40-.70
.07x= .70

Divide both sides by .07:
%28.07x%29%2F.07+=+.70%2F.07+=+70%2F7+=10 gallons whipping cream
35+-+x+=+25 gallons 2% milk.

Check:
Does .09(10) + .02(25) = .04(35)
.9 + .5 = 1.4
It checks.

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