Question

Milk and cream are mixed together for a recipe. The total volume of the mixture is 1 cup. If the milk contains 2% fat, the cream contains 18% fat, and the mixture contains 6% fat, how much cream is in the mixture?

Answer 1

6% fat is my guess.
-Nick

Answer 2

It'd be helpful to choose letters to represent the volumes of the milk and cream in the 1-cup mixture.  Why not let m=volume of milk, and c=volume of cream?
Then m+c=1 cup (total volume of mixture).

How much fat does the milk contribute?  It's 2% fat.  The amount of pure fat there is 0.02m (which has units of measurement "cups" and is a volume).  How much fat does the cream contribute?  It's 18% fat.  The amount of pure fat there is 0.18c.  The amount of pure fat in the mixture is 0.06(1 cup), or 0.06 cup.

Then 0.02m + 0.18c = 0.06 (cup).

To remove the fractions, mult. this equation by 100:  2m + 18c = 6
Remember that m + c = 1.  solving this for m, m = 1 - c.  Substituting this relationship into 2m + 18c = 6, we get 2(1-c) + 18c = 6.

Thus, 2-2c + 18 c = 6.  Solve this for c:  16c = 4; c=1/4, or c=0.25.
There is 0.25 cup (1/4 cup) of cream in the mixture, and (1-0.25) cup, or 0.75 cup, of milk in the mixture.

So, there's your answer:  "here is 0.25 cup (1/4 cup) of cream in the mixture."

It's very important that you choose and use variables to represent the unknown quantities (that is, the volumes of the milk and cream in the mix), defining them carefully.