Does (a – 2)(b + 4) = 8?

Question

(1) ab = 2b – 4a
(2) a = 6

Source- Kaplan Mock Test

4d · Answer

(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

KanishkM · Answer

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

oblivion441 · Answer

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!!!!

KanishkM · Answer

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

oblivion441 · Answer

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.

KanishkM · Answer

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 ??

Bunuel · Answer

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.

4d · Answer

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

oblivion441 · Answer

Thanks dude, 
I got it!

yashikaaggarwal · Answer

How can 0! Be 16?

Posted from my mobile device

bumpbot · Answer

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Does (a – 2)(b + 4) = 8?

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GMAT Data Sufficiency (DS) Questions

Does (a – 2)(b + 4) = 8?

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