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Does (a – 2)(b + 4) = 8?

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Does (a – 2)(b + 4) = 8?  [#permalink]

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New post 20 Jan 2019, 22:45
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Question Stats:

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Does (a – 2)(b + 4) = 8?

(1) ab = 2b – 4a
(2) a = 6


Source- Kaplan Mock Test
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Re: Does (a – 2)(b + 4) = 8?  [#permalink]

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New post 20 Jan 2019, 22:59
1
(a – 2)(b + 4) = ab + 4a -2b - 8

(1) ab = 2b – 4a
ab+4a - 2b=0
Hence (a – 2)(b + 4) = ab + 4a -2b - 8 = 0 -8 = -8
Sufficient
(2) a = 6
(a – 2)(b + 4) = 4(b+4)
Without the value of b - Insufficient
A
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Does (a – 2)(b + 4) = 8?  [#permalink]

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New post Updated on: 21 Jan 2019, 03:40
1
divyadamahe wrote:
Does (a – 2)(b + 4) = 8?

(1) ab = 2b – 4a
(2) a = 6


Source- Kaplan Mock Test


Does (a – 2)(b + 4) = 8 -------(a)

Statement 1 ab = 2b - 4a

(a) becomes ab + 4a - 2b - 8 = 8
2b - 4a + 4a -2b = 16
0 != 16
Sufficient

Statement 2, a = 6

Though if i substitute this in the question, I can calculate the value of b as - 2,

But question has asked us does (a – 2)(b + 4) = 8, is LHS = RHS

4b + 16 = 8, so now b could have been a fraction, integer or a real number.

Without the value of b we cannot tell that, Hence insufficient

Answer A
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Originally posted by KanishkM on 20 Jan 2019, 23:16.
Last edited by KanishkM on 21 Jan 2019, 03:40, edited 1 time in total.
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Re: Does (a – 2)(b + 4) = 8?  [#permalink]

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New post 21 Jan 2019, 03:18
KanishkM wrote:
divyadamahe wrote:
Does (a – 2)(b + 4) = 8?

(1) ab = 2b – 4a
(2) a = 6


Source- Kaplan Mock Test


Does (a – 2)(b + 4) = 8 -------(a)

Statement 1 ab = 2b - 4a

(a) becomes ab + 4a - 2b - 8 = 8
2b - 4a + 4a -2b = 16
0 != 16
Sufficient

Statement 2, a = 6

Though if i substitute this in the question, I can calculate the value of b as - 2, But b can be an integer, a number or a real number
Not sufficient


Answer A


Hi dude,
Can you explain to me your reasoning? If b equals -2; why am I suppose to say that the solution is not correct?

Thank you very much!!!!
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Re: Does (a – 2)(b + 4) = 8?  [#permalink]

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New post 21 Jan 2019, 03:35
oblivion441 wrote:
KanishkM wrote:
divyadamahe wrote:
Does (a – 2)(b + 4) = 8?

(1) ab = 2b – 4a
(2) a = 6


Source- Kaplan Mock Test


Does (a – 2)(b + 4) = 8 -------(a)

Statement 1 ab = 2b - 4a

(a) becomes ab + 4a - 2b - 8 = 8
2b - 4a + 4a -2b = 16
0 != 16
Sufficient

Statement 2, a = 6

Though if i substitute this in the question, I can calculate the value of b as - 2, But b can be an integer, a number or a real number
Not sufficient


Answer A


Hi dude,
Can you explain to me your reasoning? If b equals -2; why am I suppose to say that the solution is not correct?

Thank you very much!!!!


I had the same question when i solved this question, but later i realized that the question
was never
1) to find the value of b,
2) then prove that the question stem is true or not.

IMO it was like is LHS = RHS, i should like edit my explanation a bit :-)
_________________
If you notice any discrepancy in my reasoning, please let me know. Lets improve together.

Quote which i can relate to.
Many of life's failures happen with people who do not realize how close they were to success when they gave up.
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Re: Does (a – 2)(b + 4) = 8?  [#permalink]

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New post 21 Jan 2019, 03:41
KanishkM wrote:

I had the same question when i solved this question, but later i realized that the question
was never
1) to find the value of b,
2) then prove that the question stem is true or not.

IMO it was like is LHS = RHS, i should like edit my explanation a bit :-)


I understand your point. But if I know that a= 6 and so b=-2 I can answer the question (a – 2)(b + 4) = 8
The only thing that comes in my mind is that we are getting to different results, and this is usually impossible in DS questions.
But I still don't get why b=-2 is not correct.
:)
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Re: Does (a – 2)(b + 4) = 8?  [#permalink]

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New post 21 Jan 2019, 03:46
oblivion441 wrote:
KanishkM wrote:

I had the same question when i solved this question, but later i realized that the question
was never
1) to find the value of b,
2) then prove that the question stem is true or not.

IMO it was like is LHS = RHS, i should like edit my explanation a bit :-)


I understand your point. But if I know that a= 6 and so b=-2 I can answer the question (a – 2)(b + 4) = 8
The only thing that comes in my mind is that we are getting to different results, and this is usually impossible in DS questions.
But I still don't get why b=-2 is not correct.
:)


No worries, I am glad that b = -2, stirred some questions for someone, Nevertheless lets wait for the gc community to reply.

4d, What is your take at this ?

Can you also share your insight on this ??
_________________
If you notice any discrepancy in my reasoning, please let me know. Lets improve together.

Quote which i can relate to.
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Re: Does (a – 2)(b + 4) = 8?  [#permalink]

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New post 21 Jan 2019, 03:49
1
1
oblivion441 wrote:
KanishkM wrote:

I had the same question when i solved this question, but later i realized that the question
was never
1) to find the value of b,
2) then prove that the question stem is true or not.

IMO it was like is LHS = RHS, i should like edit my explanation a bit :-)


I understand your point. But if I know that a= 6 and so b=-2 I can answer the question (a – 2)(b + 4) = 8
The only thing that comes in my mind is that we are getting to different results, and this is usually impossible in DS questions.
But I still don't get why b=-2 is not correct.
:)


b = -2 is not correct because from (2) we don't get that b = -2. THE QUESTION BECOMES: does b = -2? Notice that we are NOT given that (a – 2)(b + 4) = 8. We are asked to answer this.

Does (a – 2)(b + 4) = 8?

Does ab + 4a - 2b = 16?

(1) ab = 2b – 4a --> ab + 4a - 2b = 0. So, the answer to the question is NO: ab + 4a - 2b equals to 0 not 16. Sufficient.

(2) a = 6. The question becomes: does b = -2? We don't know that. Not sufficient.

Answer: A.
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Re: Does (a – 2)(b + 4) = 8?  [#permalink]

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New post 21 Jan 2019, 04:08
1
KanishkM oblivion441

The question is Does (a – 2)(b + 4) = 8?
Now when a = 6
Is 4(b+4)=8 ?
If b=-2 , its 8 - YES
If is any other number - NO
In any DS question if you have YES and NO as your answer . Then the data is insufficient .
That means using a=6 , i cannot say whether (a – 2)(b + 4) = 8 or not
Hence insufficient
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Re: Does (a – 2)(b + 4) = 8?  [#permalink]

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New post 21 Jan 2019, 07:12
4d wrote:
KanishkM oblivion441

The question is Does (a – 2)(b + 4) = 8?
Now when a = 6
Is 4(b+4)=8 ?
If b=-2 , its 8 - YES
If is any other number - NO
In any DS question if you have YES and NO as your answer . Then the data is insufficient .
That means using a=6 , i cannot say whether (a – 2)(b + 4) = 8 or not
Hence insufficient


Thanks dude,
I got it!
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Re: Does (a – 2)(b + 4) = 8?   [#permalink] 21 Jan 2019, 07:12
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