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edit: I should add, though, that because this is a DS question, we can stop long before we reach the answer. _________________

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Re: OG C DS 131 Is 5^k less than 1000? a. 5^(k+1) >3000 b. [#permalink]

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23 Nov 2011, 11:55

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Simple Solution: 1) One variable. Inequality. Cannot determine the value of k -> Insufficiant 2) One Vasriable. One euality Equation. I can determine the value of k -> Sufficient

Hence B. I would not calculate anything. Time to solve - < 10 Secs. _________________

Re: OG C DS 131 Is 5^k less than 1000? a. 5^(k+1) >3000 b. [#permalink]

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23 Nov 2011, 12:12

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@iamgame I agree with your theory for statement 2 but I disagree for statement 1

Just because it is inequality and one variable you can't dismiss it. If after simplification you got the statement 1 as \(5^k > 1200\), it would have been sufficient.

We have to be careful in generalizing that rule. This is especially dangerous for high level inequality problems.

Re: OG C DS 131 Is 5^k less than 1000? a. 5^(k+1) >3000 b. [#permalink]

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23 Nov 2011, 14:20

Expert's post

iamgame wrote:

Simple Solution: 1) One variable. Inequality. Cannot determine the value of k -> Insufficiant 2) One Vasriable. One euality Equation. I can determine the value of k -> Sufficient

Hence B. I would not calculate anything. Time to solve - < 10 Secs.

If you're approaching DS questions in that way, you won't get very many of them right, unfortunately. For example, if you saw this question:

Is 5^k < 125?

1) k < 3 2) 5^k = 5*5^(k-1)

then Statement 1 is sufficient, since if k is less than 3, then 5^k is less than 5^3 = 125. Statement 2 gives an equation, but it is not sufficient, since it is always true - it gives you no information at all about the value of k. So in this example, the statement with the inequality *is* sufficient, and the statement with the equation is *not* sufficient. There are countless similar examples that you can find among official questions. _________________

GMAT Tutor in Toronto

If you are looking for online GMAT math tutoring, or if you are interested in buying my advanced Quant books and problem sets, please contact me at ianstewartgmat at gmail.com

Re: OG C DS 131 Is 5^k less than 1000? a. 5^(k+1) >3000 b. [#permalink]

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15 Feb 2014, 03:06

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seabhi wrote:

Hi, Please add OA. Thanks.

Done.

Is 5^k less than 1,000?

Is \(5^k<1,000\)?

(1) 5^(k+1) > 3,000 --> \(5^k>600\) --> if \(k=4\) then the answer is YES: since \(600<(5^4=625)<1,000\) but if \(k=10\), for example, then the answer is NO. Not sufficient.

(2) 5^(k-1) = 5^k - 500 --> we can solve for k and get the single numerical value of it, hence this statement is sufficient. Just to illustrate: \(5^k-5^{k-1}=500\) --> factor out \(5^{k-1}\): \(5^{k-1}(5-1)=500\) --> \(5^{k-1}=125\) --> \(k-1=3\) --> \(k=4\). Sufficient.

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