above720 wrote:
Bob just filled his car's gas tank with 20 gallons of gasohol, a mixture consisting of 5% ethanol and 95% gasoline. If his car runs best on a mixture consisting of 10% ethanol and 90% gasoline, how many gallons of ethanol must he add into the gas tank for his car to achieve optimum performance?
A. 9/10
B. 1
C. 10/9
D. 20/19
E. 2
Responding to a pm:
Using Weighted average formula:
w1/w2 = (A2 - Avg)/(Avg - A1)
You need to mix 5% ethanol mixture with 100% ethanol to give a 10% ethanol mixture.
w1/w2 = (100 - 10)/(10 - 5) = 18/1
For every 18 parts of gasohol, you need to put 1 part of pure ethanol.
So if you have 20 gallons of gasohol, you need to put 20/18 (= 10/9) gallons of pure ethanol.
Answer (C)
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Karishma
Veritas Prep GMAT Instructor
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