You know that ax + b = 0, so you can safely say that ax = -b. HOWEVER, don't make the mistake of directly dividing by a, since a might be zero (we are not told that a is not zero anywhere, so we must not assume that). Once you get to ax = -b, split it up into two cases:
a. a is zero
b. a is not zero
GMAT Prep DS problem
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Source: Beat The GMAT — Data Sufficiency |
Hi guys, I've been starting this whole "GMAT" process for the past week or so, but I feel that this entire Data Sufficiency part is still throwing me for a loop.
I think I am mainly confused about how to actually approach the questions and where to start.
They give you the first equation. Then they ask you "does something equal something if something..."
Then they give you the two possibilities. So I've read that we should take each possibility (a) and (b) as separate and to try and solve.
So if the initial equation is simplified to ax = -b,... then should I just go ahead and plug in a generic set of numbers for possibility (a) for the a+b>0?...
Sorry about the elementary question but I guess I'm just missing the order and approach of these. I noticed that on the first practice test I tried, I would get the math correct, but my DS answers were wrong because of the final determination of the answer.
Thanks!
I think I am mainly confused about how to actually approach the questions and where to start.
They give you the first equation. Then they ask you "does something equal something if something..."
Then they give you the two possibilities. So I've read that we should take each possibility (a) and (b) as separate and to try and solve.
So if the initial equation is simplified to ax = -b,... then should I just go ahead and plug in a generic set of numbers for possibility (a) for the a+b>0?...
Sorry about the elementary question but I guess I'm just missing the order and approach of these. I noticed that on the first practice test I tried, I would get the math correct, but my DS answers were wrong because of the final determination of the answer.
Thanks!
Dana,
You are right we should not divide by a .But if we take the 2 scenarios
a=0
a!=0
then stmt 1 tells that:
a+b>0 then
if a is zero then x is -b/0 which is not possible, so lets take a with +ve or -ve number then we would get different values for ax. So Stmt 1 is not sufficient
If we take stmt 2 : a-b>0 then
If a is zero then x is -b/0 which is not possible, on the other hand if a is not zero then say a could be +ve or -ve then we would get different values for ax. So insufficient.
So answer is E.
You are right we should not divide by a .But if we take the 2 scenarios
a=0
a!=0
then stmt 1 tells that:
a+b>0 then
if a is zero then x is -b/0 which is not possible, so lets take a with +ve or -ve number then we would get different values for ax. So Stmt 1 is not sufficient
If we take stmt 2 : a-b>0 then
If a is zero then x is -b/0 which is not possible, on the other hand if a is not zero then say a could be +ve or -ve then we would get different values for ax. So insufficient.
So answer is E.
I believe the is one "flashcard" directly from this site that mentions for DS questions that to solve for n unknown variables, you must have presented at least as many options to solve for that variable?? (perhaps I was missing the idea of this rule??)
Other than than, wouldn't the easiest approach to this question just be to plug in numbers for each possibility until you realize that they are BOTH not sufficient?
Other than than, wouldn't the easiest approach to this question just be to plug in numbers for each possibility until you realize that they are BOTH not sufficient?
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DanaJ
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Yes, I think that would be a good strategy for this question. Remember however that picking numbers is not always the best choice. In my experience, number picking works best when you are supposed to give counterexamples - like in this question.
