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16 Sep 2014, 00:44
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A single filter reduces the concentration of harmful substances in water by half. If water with less than 0.1 percent harmful substances is deemed clean, minimum of how many filters are needed to purify 200 liters of water that contains 0.5 liters of these substances?
A. 2
B. 3
C. 5
D. 6
E. 8
A. 2
B. 3
C. 5
D. 6
E. 8
16 Sep 2014, 00:44
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Official Solution:
A single filter reduces the concentration of harmful substances in water by half. If water with less than 0.1 percent harmful substances is deemed clean, minimum of how many filters are needed to purify 200 liters of water that contains 0.5 liters of these substances?
A. 2
B. 3
C. 5
D. 6
E. 8
At the start, the concentration of harmful substances in the water is \(\frac{0.5}{200}*100\% = 0.25\%\). Using one filter brings this concentration down to \(0.125\%\). After deploying two filters, the concentration falls further to \(0.0625\%\), which is below the \(0.1\%\) threshold.
Therefore, a minimum of two filters is required to bring the concentration of harmful substances below the acceptable limit.
Answer: A
A single filter reduces the concentration of harmful substances in water by half. If water with less than 0.1 percent harmful substances is deemed clean, minimum of how many filters are needed to purify 200 liters of water that contains 0.5 liters of these substances?
A. 2
B. 3
C. 5
D. 6
E. 8
At the start, the concentration of harmful substances in the water is \(\frac{0.5}{200}*100\% = 0.25\%\). Using one filter brings this concentration down to \(0.125\%\). After deploying two filters, the concentration falls further to \(0.0625\%\), which is below the \(0.1\%\) threshold.
Therefore, a minimum of two filters is required to bring the concentration of harmful substances below the acceptable limit.
Answer: A
10 Jan 2016, 17:13
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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.
