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Weighted Average

by Priyaranjan » Thu Dec 03, 2015 8:36 am
A feed store sells two varieties of birdseed: Brand A which is 40% millet and 60% sunflower, and Brand B, which is 65% millet and 35% sunflower. If a customer purchases a mix of the two types of birdseed that is 50% millet, what percent of the mix is Brand A?
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by GMATGuruNY » Thu Dec 03, 2015 8:39 am
Priyaranjan wrote:A feed store sells two varieties of birdseed: Brand A which is 40% millet and 60% sunflower, and Brand B, which is 65% millet and 35% sunflower. If a customer purchases a mix of the two types of birdseed that is 50% millet, what percent of the mix is Brand A?
A's millet percentage: 40%.
B's millet percentage: 65%.
Millet percentage in the mixture: 50%.

The following approach is called ALLIGATION -- a very efficient way to handle MIXTURE PROBLEMS.

Step 1: Plot the 3 percentages on a number line, with the percentages for A and B on the ends and the percentage for the mixture in the middle.
A 40%-----------50%-----------65% B

Step 2: Calculate the distances between the percentages.
A 40%----10-----50%----15-----65% B

Step 3: Determine the ratio in the mixture.
The required ratio of A to B is equal to the RECIPROCAL of the distances in red.
A:B = 15:10 = 3:2.

Since A:B = 3:2, every 5 pounds of mixture is composed of 3 pounds of A and 2 pounds of B.
Thus:
A/total = 3/5 = [spoiler]60%[/spoiler].

For two similar problems, check here:

https://www.beatthegmat.com/ratios-fract ... 115365.htm
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by [email protected] » Thu Dec 03, 2015 9:57 am
Hi Priyaranjan,

This prompt is a Weighted Average question, so you can set up the Average Formula and work through it algebraically if you like.

We're told about 2 Brands of seed:
1) Brand A = 40% millet and 60% sunflower
2) Brand B = 65% millet and 35% sunflower

We're told that mixing a certain amount of each led to a 50% millet mixture. We're asked for the PERCENT of this final mix that is BRAND A.

To start, here's the formula:

A = number of pounds of A
B = number of pounds of B

[.4(A) + .65(B)] / (A+B) = .5

.4A + .65B = .5A + .5B
.15B = .1A
15B = 10A
3B = 2A

3/2 = A/B

This means that for every 3 pounds of A, there are 2 pounds of B. Out of every 5 total pounds, 3 of those pounds are A.

3/5 = 60%

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