percent butterfat in milk mixture

[Richay]

When Ryan and Lisa finished milking, they found that they had 900 pounds of milk that was 2 percent butterfat. How much butterfat did they have to add to raise the butterfat content to 10 percent? (Whole milk is a mixture of skim milk and butterfat.)

My steps.

900 / 2% = 18 pounds is butterfat

They need 10 of butterfet percent so..

18 x 4 = 72
OR
900 / 10 = 90 - 18 = 72

If you add 72+18 you get 90. And 900 / 10 = 90.

So you need 72 more butterfat pounds to get it up to 10%. So why does it say i'm wrong?

 

[JakeD]

After you add 72 pounds, you won't have 900 pounds anymore, you'll have 972 pounds with 90 pounds of butterfat. That's 9.25% butterfat.

 

[Richay]

I c, thanks

But in that case, this problems too hard

 

[JakeD]

The equation you need to solve is

\(\displaystyle \large .02 \times 900 + x = .10 (900+x)\)

where \(\displaystyle x\) is the amount of butterfat to add.

 

[Richay]

OH, the answer is 80 then.

Thanks for the help