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At a speed of 50 miles per hour

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agha79
 Post subject: At a speed of 50 miles per hour
 Post Posted: Tue Feb 15, 2011 6:36 pm 
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Posts: 98
At a speed of 50 miles per hour, a certain car uses 1 gallon of gasoline every 30 miles. If the car starts with a full 12 gallon tank of gasoline and travels for 5 hours at 50 miles per hour, the amount of gasoline used would be what fraction of a full tank?

A. 3/25
B. 11/36
C. 7/12
D. 2/3
E. 25/36

My approach to answer was that the total distance in miles traveled by the car is 250 miles (5hours*250miles/hour). If 1 gallon gasoline is used for 30 miles then for 250 miles it would be 8.33 gallons. Then 8.33/12 gives approximately 2/3, which is the wrong answer. OA is E. Can someone help where I went wrong?
Thanks.
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bharathholla
 Post subject: Re: At a speed of 50 miles per hour
 Post Posted: Thu Feb 17, 2011 5:43 am 
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You almost got to the answer.
So the fraction of gas used is (250/30)*(1/12) = 25/36. Since the answer choices are represented as fractions I wouldn't bother calculating what 250/30 is.

Hope this helps.
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agha79
 Post subject: Re: At a speed of 50 miles per hour
 Post Posted: Thu Feb 17, 2011 7:48 am 
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Posts: 98
Thanks. That was an extremely silly mistake. The question asked for fraction so I shouldn’t have converted gallons into decimals :-(
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jmiceli0819
 Post subject: Re: At a speed of 50 miles per hour
 Post Posted: Thu Feb 17, 2011 5:22 pm 
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Posts: 15
Hi,

Can you explain why you multiplied by 1/12 i did long division and got the correct answer.
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jnelson0612
 Post subject: Re: At a speed of 50 miles per hour
 Post Posted: Thu Feb 17, 2011 6:38 pm 
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ManhattanGMAT Staff


Posts: 1857
jmiceli0819 wrote:
Hi,

Can you explain why you multiplied by 1/12 i did long division and got the correct answer.


We find that the car used 8 1/3 gallons of gas out of the 12 available in the tank. The question asks what fraction 8 1/3 is of 12. To express this I convert 8 1/3 to 25/3 and then put 25/3 over 12 like this:

25
3__
12
1

I need to flip the 12/1 and multiply, so I end up multiplying 25/3 by 1/12. Hope this helps!

_________________
Jamie Nelson
ManhattanGMAT Instructor
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jmiceli0819
 Post subject: Re: At a speed of 50 miles per hour
 Post Posted: Thu Feb 17, 2011 8:27 pm 
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Posts: 15
oh wow ... i cant believe i didnt see this before!!!!
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jnelson0612
 Post subject: Re: At a speed of 50 miles per hour
 Post Posted: Sat Feb 19, 2011 10:34 pm 
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ManhattanGMAT Staff


Posts: 1857
Great! :-)

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Jamie Nelson
ManhattanGMAT Instructor
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