This is a weighted averages problem; normally, you find the average by adding up all the numbers and dividing the total by the number of quantities you added up:
(X + Y + Z) / 3 = Average
Thanks to the distributive property, I can divide each value by 3:
X/3 + Y/3 + Z/3 = Average
Each piece (X, Y, and Z) gets an equal share in composing the average. They each get one third or 33% of creating the final average. In this problem, the 6% butterfat milk and the 3% butterfat milk, don't have an equal percentage in making the 5% milk. I will set up the problem like so:
(percentage of contribution from 6% milk)(6% milk) + (percentage of contribution from 3% milk)(3% milk) = 5% milk
Now, how do I find out how much of a contribution each milk will make?
I know that there will be 30 quarts of milk in all, so....
(quarts of 6% milk) + (quarts of 3%) = 30
Now, I will call the quarts of 6% milk X
I will call the quarts of 3% milk Y
(quarts of 6% milk) + (quarts of 3%) = 30
X + Y = 30
I know that 30 is the total amount of quarts. To find how much "weight" each milk has in computing the average, I can take each amount of milk and divide it by the total amount of milk:
(contribution from 6% milk) = X/30
(contribution from 3% milk) = Y/30
As fractions, these numbers represent how much weight each milk will have in computing the average.
X/30(6% milk) + Y/30(3% milk) = 5% milk
Now, in order to do the necessary math, I need to convert all percentages into decimals
X/30(0.06) + Y/30(0.03) = 0.05
Since both the X and the Y are being divided by 30, I can multiply both sides of the equation by 30:
X(0.06) + Y(0.03) = 0.05*30 (which equals 1.5)
Now, we still have two unknown numbers. I can get rid of one of them by using our other equation:
X + Y = 30 (because the total number of quarts will equal 30)
and I can simply rearrange it thus:
X = 30 - Y
And I can substitute 30 - Y in for X:
(30 - Y)(0.06) + Y(0.03) = 1.5
This is what Mark set up for you; just solve for the amount of 3% milk and then find the amount of 6% milk.
:)
Mark M.
06/28/16