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Salad dressing A is made up of 30% vinegar and 70% oil...

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Salad dressing A is made up of 30% vinegar and 70% oil...

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Salad dressing A is made up of 30% vinegar and 70% oil, and salad dressing B contains 10% vinegar and 90% oil. If the 2 dressing are combined to produce a salad dressing that is 15% vinegar, dressing A comprises what % of the new dressing?

A. 15%
B. 20%
C. 25%
D. 40%
E. 55%

The OA is C.

Please, can any expert explain this PS question for me? I can't get the correct answer. I need your help. Thanks.

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swerve wrote:
Salad dressing A is made up of 30% vinegar and 70% oil, and salad dressing B contains 10% vinegar and 90% oil. If the 2 dressing are combined to produce a salad dressing that is 15% vinegar, dressing A comprises what % of the new dressing?

A. 15%
B. 20%
C. 25%
D. 40%
E. 55%
Vinegar in A = 30%.
Vinegar in B = 10%.
Vinegar in the MIXTURE of A and B = 15%.

To determine the ratio of A to B, use 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 (30% and 10%) on the ends and the percentage for the mixture (15%) in the middle.
A 30----------15-----------10 B

Step 2: Calculate the distances between the percentages.
A 30----15----15----5----10 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 = 5:15 = 1:3.

Thus, of every 4 liters:
A = 1 liter.
B = 3 liters.
A/total = 1/4 = 25%.

The correct answer is C.

Other alligation problems:
http://www.beatthegmat.com/mixture-problem-rabbit-proteins-t190121.html#593241

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swerve wrote:
Salad dressing A is made up of 30% vinegar and 70% oil, and salad dressing B contains 10% vinegar and 90% oil. If the 2 dressing are combined to produce a salad dressing that is 15% vinegar, dressing A comprises what % of the new dressing?

A. 15%
B. 20%
C. 25%
D. 40%
E. 55%
We can let the amount of dressing A = a and the amount of dressing B = b, thus:

0.3a + 0.1b = 0.15(a + b)

30a + 10b = 15a + 15b

15a = 5b

3a = b

So dressing A is a/(a + b) = a/(a + 3a) = 1/4 = 25% of the new dressing.

Answer: C

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Jeffrey Miller Head of GMAT Instruction

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