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Manhattan gmat DS question - doesn't make sense

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by sk123 » Wed Feb 25, 2009 10:12 am
Does anyone get this manhattan gmat question. I tried to understand their reason but it didn't make any sense ... just wanna see if anyone has another way of doing it.

If a^2 is not equal to b^2, what is the value of

a - b
__________ ?
a^2 – b^2



(1) 3^b + 2 = 81

(2) a = 3^2b – 3
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Source: — Data Sufficiency |
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by ontopofit » Wed Feb 25, 2009 11:36 am
IMO C
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by sk123 » Wed Feb 25, 2009 11:52 am
Here's the answer:

(1) SUFFICIENT: We can rewrite this equation in a base of 3: 3b + 2 = 34, which means that b + 2 = 4 and therefore b = 2.

We can plug this value into the equation a = 3b – 1 to solve for a.

(2) SUFFICIENT: We can set the right side of this equation equal to the right side of the equation in the question (both sides equal a). 3b – 1 = 32b – 3, which means that b – 1 = 2b – 3 and therefore b = 2.

We can plug this value into the equation a = 3b – 1 to solve for a.

The correct answer is D.
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by schumi_gmat » Wed Feb 25, 2009 12:14 pm
we have to find 1/(a+b). The a-b can be cancelled as a-b not equal to 0

From A) 3^(b+2) = 81, i.e. b+2=4 and b=2

We dont know value of a. Hence insuff

From B) a= 3^2b-3

one equation, 2 variables. Hence insuff

Hence ans is C.
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by sk123 » Wed Feb 25, 2009 3:48 pm
The answer I posted above is from Manhattan gmat ... from one of the online exams ...
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by mygmat.2009 » Wed Feb 25, 2009 6:41 pm
I agree with schumi_gmat , unless sky123 is missing to give us part of the question!

Sky123, in your answer you mention equation [a = 3b – 1], yet the question you posted does not mention this equality.


Sam
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by gabriel » Thu Feb 26, 2009 9:49 am
sk123 wrote:Here's the answer:

(1) SUFFICIENT: We can rewrite this equation in a base of 3: 3b + 2 = 34, which means that b + 2 = 4 and therefore b = 2.

We can plug this value into the equation a = 3b – 1 to solve for a.

(2) SUFFICIENT: We can set the right side of this equation equal to the right side of the equation in the question (both sides equal a). 3b – 1 = 32b – 3, which means that b – 1 = 2b – 3 and therefore b = 2.

We can plug this value into the equation a = 3b – 1 to solve for a.

The correct answer is D.
Where did you get a=3b-1 from?, it is not part of the question. So if the question is correct then schumi_gmat is correct.
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by sk123 » Thu Feb 26, 2009 1:44 pm
That's what I was trying to get at ... that is the official answer from the manhattan gmat exam. I was thinking I might be missing something but I guess their expl doesn't add up.
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by Uri » Thu Feb 26, 2009 2:17 pm
utterly confused! first i thought MGMAT has used technique to convert decimal number to a different base (base of 3 in stead of base of 10). but after following more carefully i understood that it was not so. don't understand from where they have got the other equation.

sk123, if you enclosed the exponents within brackets, that would have been better for understanding. but any way, this solution by MGMAT is very confusing.

if the question and the solution is reproduced correctly, then can any MGMAT tutor available here please help us?
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by fleshins » Sat Feb 28, 2009 10:24 pm
how does this make any sense either?

"We can rewrite this equation in a base of 3: 3b + 2 = 34, which means that b + 2 = 4 and therefore b = 2."

Thanks for the explanation, if there is one ...
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