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# M11-15

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Math Expert
Joined: 02 Sep 2009
Posts: 59712
M11-15  [#permalink]

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16 Sep 2014, 00:44
00:00

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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Math Expert
Joined: 02 Sep 2009
Posts: 59712
Re M11-15  [#permalink]

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16 Sep 2014, 00:44
1
1
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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Manager
Joined: 05 Jul 2015
Posts: 93
Concentration: Real Estate, International Business
GMAT 1: 600 Q33 V40
GPA: 3.3
Re M11-15  [#permalink]

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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.
Intern
Joined: 14 Mar 2015
Posts: 16
Location: India
Concentration: General Management, Strategy
Schools: ISB '18, IIMA , IIMB
GPA: 3.53
WE: Engineering (Other)
Re: M11-15  [#permalink]

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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
Manager
Joined: 06 Sep 2018
Posts: 171
Location: Pakistan
Concentration: Finance, Operations
GPA: 2.87
WE: Engineering (Other)
Re: M11-15  [#permalink]

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

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