Is a = b c? (1) c = a + b (2) a < 0 : GMAT Data Sufficiency (DS)
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# Is a = b c? (1) c = a + b (2) a < 0

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Is a = b c? (1) c = a + b (2) a < 0 [#permalink]

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22 Jul 2010, 12:06
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Is ┃a┃ = b – c?

(1) c = a + b

(2) a < 0
[Reveal] Spoiler: OA

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22 Jul 2010, 12:29
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aiming4mba wrote:
Is ┃a┃ = b – c?

(1) c = a + b

(2) a < 0

If $$a<0$$ it must be true that $$-a=b-c$$;
If $$a\geq{0}$$ it must be true that $$a=b-c$$.

(1) $$c=a+b$$ --> $$-a=b-c$$. But we don't know whether $$a<0$$. Not sufficient.
(2) $$a<0$$. Question becomes is $$-a=b-c$$? We don't know that. Not sufficient.

(1)+(2) Form 2: $$a<0$$, question becomes is $$-a=b-c$$? Statement 1 confirms this. Sufficient.

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22 Jul 2010, 14:25
aiming4mba wrote:
Is ┃a┃ = b – c?

(1) c = a + b

(2) a < 0

why not (A)?

-a = b-c
therefore b-c = |a|......
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22 Jul 2010, 15:06
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Quote:
why not (A)?

-a = b-c
therefore b-c = |a|......

If b-c=-a, then lal=-(b-c), or c-b=lal, which does not answer our original question, b-c=lal.

Thanks,

Jared
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Re: Is a = b c? (1) c = a + b (2) a < 0 [#permalink]

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10 Oct 2012, 11:06
Is this approach correct?

For ┃a┃ = b – c to be true , (b - c ) >=0 ie positive

(1) c = a + b : for (b - c ) >=0 to be true a needs to be <=0. Not given, hence insufficient
(2) a < 0, says nothing about (b - c ) >=0 Hence insufficient
(1)+(2) proves b - c>=0. Sufficient.
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Re: Is a = b c? (1) c = a + b (2) a < 0 [#permalink]

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24 May 2016, 14:12
Hi Bunuel
Actually, I did not understand the concept of statement 2 confirms statement 1.
if the two statements confirmed each other, it means that answer C.
please clarify more regarding the concept. I always fall in the traps of absolute value DS questions.
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Re: Is a = b c? (1) c = a + b (2) a < 0 [#permalink]

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22 Sep 2016, 21:59
Hi Bunuel, I know this is an old question, but if statement 2 says a is negative can we not answer it because we plug the negative a back into the absolute value in the stem? My thought was that since lal=b-c is either a=b-c or -a=c-b we could say b is sufficient since statement 2 says a is negative. I just don't fully understand the concept.
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Re: Is a = b c? (1) c = a + b (2) a < 0 [#permalink]

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22 Sep 2016, 23:19
fiel9882 wrote:
Hi Bunuel, I know this is an old question, but if statement 2 says a is negative can we not answer it because we plug the negative a back into the absolute value in the stem? My thought was that since lal=b-c is either a=b-c or -a=c-b we could say b is sufficient since statement 2 says a is negative. I just don't fully understand the concept.

The question asks: Is ┃a┃ = b – c.

From (2) we know that a is negative. How this can be sufficient to answer the question whether┃a┃ = b – c?
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Re: Is a = b c? (1) c = a + b (2) a < 0 [#permalink]

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23 Feb 2017, 05:07
Prompt analysis
a ,b, c are real numbers.

Superset
The answer to this question will be either yes or no.

Translation
In order to find the answer, we need:
1# exact value of a, b, c
2# condition or equation, so that we can infer the answer.

Statement analysis
St 1: c = a+b or b-c = -a. The condition might be true if a is negative. But nothing as such is given. INSUFFICIENT

St 2: a<0. Nothing can be said about b and c. Insufficient.
ST 1 &St 2: if a<0, that mean -a is positive and b-c is positive.ANSWER

Option C
Re: Is a = b c? (1) c = a + b (2) a < 0   [#permalink] 23 Feb 2017, 05:07
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