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Dont understand the answer to this

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Dont understand the answer to this

by edelauna » Mon Mar 14, 2011 5:50 pm
based on the attached image, the prep gmat test is telling me the answer is both 1) and 2) are required for the answer however they are both telling me that x > y , and in each case x and y can be negative or positive, i do not understand the answer, could someone please help in explaining this to me.
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Source: — Data Sufficiency |
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by Rich@VeritasPrep » Mon Mar 14, 2011 7:36 pm
Hey edelauna,

Unfortunately, you can't go from x/y > 1 to x>y, because you don't know whether y is positive or negative (and thus you don't know whether or not the sign will flip).

Statement 1 tells you that x > y though, but it doesn't tell you their signs.

Statement 2 tells you x/y > 1, which means that either x<y (which would happen if x and y were both negative, for example x=-2, y=-1), or x>y (which would happen if x and y were both positive). We don't know which it is.

Together, though, we know it must be true that x>y and x/y > 1, and thus x and y are both positive.

Let me know if you have any other questions!
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by Night reader » Mon Mar 14, 2011 7:39 pm
st(1) x=1/2+y Not Sufficient, since we don't know the signs of x and y, they could be both -ve OR +ve;
st(2) x/y>1, |x|>|y|; x and y can be both -ve OR +ve, hence Not Sufficient;

Combined st(1) x=1/2+y AND st(2) x/y>1
from st(1&2) if (1/2+y)/y>1 then (1/2y)>0 Hence 2y>0 AND y>0
from st(1) x=1/2 +y Hence with positive y ---> x=1/2 + y {positive value of y} We can answer Not only x and y are positive (required for this question) BUT also x>y -Hence Sufficient

answer C
Are x,y>0?
st(1) 2x-2y=1
st(2) x/y>1
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