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A chemist is mixing a solution of acetone and water. She

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A chemist is mixing a solution of acetone and water. She [#permalink]  02 Jun 2018, 03:46
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Question Stats:

62% (01:22) correct 37% (01:02) wrong based on 8 sessions
A chemist is mixing a solution of acetone and water. She currently has 30 ounces mixed, 10 of which are acetone. How many ounces of acetone should she add to her current mixture to attain a 50/50 mixture of acetone and water if no additional water is added?

(A) 2.5
(B) 5
(C) 10
(D) 15
(E) 20
[Reveal] Spoiler: OA

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Re: A chemist is mixing a solution of acetone and water. She [#permalink]  05 Jun 2018, 09:08
Expert's post
sandy wrote:
A chemist is mixing a solution of acetone and water. She currently has 30 ounces mixed, 10 of which are acetone. How many ounces of acetone should she add to her current mixture to attain a 50/50 mixture of acetone and water if no additional water is added?

(A) 2.5
(B) 5
(C) 10
(D) 15
(E) 20

We can let n = the additional amount of acetone added and create the equation:

(10 + n)/(30 + n) = 1/2

2(10 + n) = 1(30 + n)

20 + 2n = 30 + n

n = 10

Answer: C
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GRE Prep Club Legend
Joined: 07 Jun 2014
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Kudos [?]: 1782 [0], given: 397

Re: A chemist is mixing a solution of acetone and water. She [#permalink]  14 Jul 2018, 04:28
Expert's post
Explanation

The chemist now has 10 ounces of acetone in a 30-ounce mixture, so she must have 20 ounces of water.

The question ask how many ounces of acetone must be added to make this mixture a 50% solution. No additional water is added, so the solution must finish with 20 ounces of water.

Therefore, she also needs a total of 20 ounces of acetone, or 10 more ounces than the mixture currently contains.

Note that one trap answer is (B), or 5. This answer is not correct because the final number of ounces in the solution is not 30; when the chemist adds acetone, the amount of total solution also increases— adding 5 ounces acetone would result in a solution that is $$\frac{15}{30+5}$$acetone, which is not equivalent to a 50% mixture.
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Re: A chemist is mixing a solution of acetone and water. She   [#permalink] 14 Jul 2018, 04:28
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