GoBlue wrote:30 gallons of milk and water contains 20 % water in it. How many gallons of the mixture should be replaced with pure milk to bring down the percentage of water in mixture to 5 %.
A. 90
B. 15
C. 22.5
D. 18
E. 30
Answer with explaination. Also, according to your experience rate it as 500 level, 600 level or 700 level.
Here's an efficient approach that requires very little arithmetic:
We currently have .2*30 = 6 gallons of water in the mixture.
We want the amount of water to be reduced to .05 * 30 = 1.5.
So we need to lose 6 - 1.5 = 4.5 gallons of water.
Which answer choice (the amount of pure milk to be added) will reduce the amount of water by 4.5 gallons?
Since the mixture is 20% water, 20% of each answer choice will go toward replacing the amount of water currently in the mixture.
So let's take 20% of each answer choice (the amount of pure milk to be added) to see which gives us 4.5 (the amount of water we need to lose):
Answer choice C:
.2 * 22.5 = 4.5 Success!
The correct answer is C.
Last edited by
GMATGuruNY on Tue Sep 14, 2010 6:27 am, edited 3 times in total.
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