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Trouble on understanding an answer--please help, thanks!

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mlovering456
 Post subject: Trouble on understanding an answer--please help, thanks!
 Post Posted: Sat Nov 05, 2011 1:33 pm 
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Students


Posts: 2
What is the value of x?

(1) x + 2y = 4

(2) 5x + y + 15 = 3x – 3y + 23

The answer is E--but I don't understand why it cannot be C? Since I can find the value for Y in the first equation and plug it into the second equation (or vice versa) thus yielding the value for x-- The answer explanation did not address this possible solution--thanks in advance for the help!
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jssaggu.tico
 Post subject: Re: Trouble on understanding an answer--please help, thanks!
 Post Posted: Sun Nov 06, 2011 5:47 pm 
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Course Students


Posts: 15
(1) x + 2y = 4

Statement 1: It's insufficient because of 2 variables in one equation. 'x' can't be extracted. So, eliminate A and D.

(2) 5x + y + 15 = 3x – 3y + 23
=> 5x-3x+y+3y = 23-15
=> 2x+4y = 8
=> x+2y = 4 (Same as eqn in statement 1)

Statement 2: It's insufficient because of 2 variables in one equation. 'x' can't be extracted. So, eliminate B.

Both together can't be used to solve because both equations are identical. Hence, no solution.

E is the answer.
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tim
 Post subject: Re: Trouble on understanding an answer--please help, thanks!
 Post Posted: Wed Nov 23, 2011 2:05 pm 
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ManhattanGMAT Staff


Posts: 2242
Location: Southwest Airlines, seat 21C
Thanks navjot. Remember, if you want to prove something insufficient, all you have to do is find two possibilities for an answer. If you have two identical linear equations, then x can take on literally any real value. Try it for yourself if you don’t see that..

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Tim Sanders
Manhattan GMAT Instructor
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