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Each week we'll be posting several questions from The Official Guide For GMAT® Quantitative Review, 2ND Edition and then after couple of days we'll provide Official Answer (OA) to them along with a slution.

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Re: If n is a positive integer and k + 2 = 3^n, which of the fol [#permalink]

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17 Mar 2014, 04:31

k+2 = 3^n Simply Substitute

1+2 = 3 = 3^1 4+2 = 6 Not a power of 3 so rejected and hence, the answer 7+2 = 9 = 3^2 25+2 = 27 = 3^3 79 + 2 = 81 = 3^4 Only catch here is that even after looking at 6 you may not realize its the answer bcoz it is a multiple of 3

Re: If n is a positive integer and k + 2 = 3^n, which of the fol [#permalink]

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28 Aug 2016, 20:46

1

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One way to solve this problem. Since n is a pos int., this means that 3^n will always be odd (note that this is true even if n =0). Also 3^n-2 will always be odd since odd - even is always odd. So even values are not possible. Take out 4 since it could never be possible and that is our answer given the question

Re: If n is a positive integer and k + 2 = 3^n, which of the fol [#permalink]

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08 Jul 2017, 09:19

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In such questions, I usually follow simple approach - find the pattern, and then substitute Lets see here, what does k + 2 = 3^n means ?

This means K is a number which is nothing but (some) power of 3 - 2. (powers of 3 = 3, 9, 27, 81) Now let us substitute the answers, and ans will be straightforward. (A) 1 = 3 - 2 it works (B) 4 = this does not work (C) 7 = 9 - 2. it works too (D) 25 = 27 - 2 this does not work (E) 79 = 81 - 2 . this worls