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M11-15

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M11-15  [#permalink]

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New post 16 Sep 2014, 00:44
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Difficulty:

  45% (medium)

Question Stats:

60% (01:06) correct 40% (00:56) wrong based on 107 sessions

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One filter decreases the concentration of harmful substances in water by half. If water that contains less than 0.1 percent of harmful substances is considered clean, how many filters are required to clean 200 liters of water containing 0.5 liters of harmful substances ?

A. 2
B. 3
C. 5
D. 6
E. 8

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Re M11-15  [#permalink]

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New post 16 Sep 2014, 00:44
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Official Solution:

One filter decreases the concentration of harmful substances in water by half. If water that contains less than 0.1 percent of harmful substances is considered clean, how many filters are required to clean 200 liters of water containing 0.5 liters of harmful substances ?

A. 2
B. 3
C. 5
D. 6
E. 8

Initially, the concentration of harmful substances in the water is \(\frac{0.5}{200}*100\% = 0.25\%\). One filter will reduce this concentration to \(0.125\%\). Two filters will reduce the concentration to \(0.0625\%\) which is less than \(0.1\%\).

Answer: A
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Re M11-15  [#permalink]

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New post 10 Jan 2016, 17:13
I think this is a high-quality question and I agree with explanation. I'm not a GMAT expert but this question definitely does not seem like a 700 level question.
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Re: M11-15  [#permalink]

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New post 28 Jun 2016, 17:43
0.1 in 100 is clean. So 0.2 in 200 is clean. Target is to reach 0.2 from 0.5
Answer is (A) 2
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Re: M11-15  [#permalink]

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New post 26 Jan 2019, 01:55
I think the wording of question should be "minimum how many filters are required to clean 200 liters of water containing 0.5 liters of harmful substances?" instead of "how many filters are required to clean 200 liters of water containing 0.5 liters of harmful substances?" because in current situation all answers could satisfy question statement.
Bunuel wrote:
Official Solution:

One filter decreases the concentration of harmful substances in water by half. If water that contains less than 0.1 percent of harmful substances is considered clean, how many filters are required to clean 200 liters of water containing 0.5 liters of harmful substances ?

A. 2
B. 3
C. 5
D. 6
E. 8

Initially, the concentration of harmful substances in the water is \(\frac{0.5}{200}*100\% = 0.25\%\). One filter will reduce this concentration to \(0.125\%\). Two filters will reduce the concentration to \(0.0625\%\) which is less than \(0.1\%\).

Answer: A
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Re: M11-15   [#permalink] 26 Jan 2019, 01:55
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