Manhattan gmat DS question - doesn't make sense

[sk123]

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

[ontopofit]

IMO C

[sk123]

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.

[schumi_gmat]

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.

[sk123]

The answer I posted above is from Manhattan gmat ... from one of the online exams ...

[mygmat.2009]

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

[gabriel]

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.

[sk123]

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.

[Uri]

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?

[fleshins]

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 ...