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What is the value of b ?

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by nikhilsrl » Sun Feb 20, 2011 7:20 am
What is the value of b ?

(1) b4 = 16

(2) 7^(3b− 4) = 49

I expected the answer to be E:

evaluating (1) b4 = 16, b could be 2 or -2 hence not sufficient

evaluating (2) we can write this as 7^(3b− 4) = 7^2 or 7^(3b− 4) = (-7)^2

This problem is taken from Kaplan CAT and the solution does not seem to consider 7^(3b− 4) = (-7)^2.

OA is B
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Source: — Data Sufficiency |
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by Geva@EconomistGMAT » Sun Feb 20, 2011 7:26 am
What you say is true, but irrelevant. 7^2 is indeed equal to -7^2, but you weren't asked for the value of the base - you were asked for the value of b.

for stat (2), whether the base is 7 or -7, in any case 3b-4 equals 2, from which you can deive a single value of b.
nikhilsrl wrote:What is the value of b ?

(1) b4 = 16

(2) 7^(3b− 4) = 49

I expected the answer to be E:

evaluating (1) b4 = 16, b could be 2 or -2 hence not sufficient

evaluating (2) we can write this as 7^(3b− 4) = 7^2 or 7^(3b− 4) = (-7)^2

This problem is taken from Kaplan CAT and the solution does not seem to consider 7^(3b− 4) = (-7)^2.

OA is B
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by stormier » Sun Feb 20, 2011 7:28 am
nikhilsrl wrote:What is the value of b ?

(1) b4 = 16

(2) 7^(3b− 4) = 49

I expected the answer to be E:

evaluating (1) b4 = 16, b could be 2 or -2 hence not sufficient

evaluating (2) we can write this as 7^(3b− 4) = 7^2 or 7^(3b− 4) = (-7)^2

This problem is taken from Kaplan CAT and the solution does not seem to consider 7^(3b− 4) = (-7)^2.

OA is B
(2) whether you write 49 as (-7)^2 or (+7)^2; the value of 7^(3b-4) can be equal to 49 if and only if 3b-4 = 2, which implies that b = +2 => Sufficient => Answer B
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