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tollbooths less than 10 miles

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AmunaGmat
 Post subject: tollbooths less than 10 miles
 Post Posted: Thu Jan 12, 2012 4:24 am 
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Course Students


Posts: 28
On her way home from work, janet drives through several toolbooths. Is there a pair of these tollbooths that are less than 10 miles apart?

1. The first and the last tollbooths are 25 miles apart.

2. Janet drives through 4 tollbooths on her way from work.

I am confused about C and E in this question. Gmatprep's answer is C.

What is the highlight in this question that the distance between the towns is additive or cumulative? To me, they can all be 10 miles apart. When I got this question in GMATprep it reminded me of 1) Question 40 DS, OG12 page 276. In this question before I looked at numbers correctly, i thought the distance from R to U is additive. 2) The RTD question 104 DS, OG12 page 282, if I am not told that the average speed of the train was constant, never assume.

Why does the same reasoning not work here?

Cheers!!!
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tim
 Post subject: Re: tollbooths less than 10 miles
 Post Posted: Sun Jan 15, 2012 4:19 pm 
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ManhattanGMAT Staff


Posts: 2242
Location: Southwest Airlines, seat 21C
if the tollbooths are all 10 miles or more apart, you have to have 3 gaps in between 4 tollbooths, which means 30 miles or more. once we bring in statement 1, this is impossible. so some of the booths must be closer than 10 miles apart..

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Tim Sanders
Manhattan GMAT Instructor
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rachelhong2012
 Post subject: Re: tollbooths less than 10 miles
 Post Posted: Sat Jan 28, 2012 5:34 pm 
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Course Students


Posts: 47
There's an easy way to do this type of problem if you turn it into an equation and approach it algebraically.

assuming you're debating between C and E, which you are, combine these two statements. You can draw a line, put down four dots, and name each gap between two dots as X, Y, Z.

So you have this equation:

X+Y+Z = 25

Test different combinations of #'s for these variables and you will see that there's no way you can have a variable that is NOT less than 10.
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tim
 Post subject: Re: tollbooths less than 10 miles
 Post Posted: Mon Jan 30, 2012 2:12 am 
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ManhattanGMAT Staff


Posts: 2242
Location: Southwest Airlines, seat 21C
thanks!

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Tim Sanders
Manhattan GMAT Instructor
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