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Re: What is the value of the integer n....Exponents [#permalink]
30 Dec 2010, 16:45

5

This post received KUDOS

Expert's post

3

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ajit257 wrote:

What is the value of the integer n?

(1) n(n + 2) = 15 (2) (n + 2)^n = 125

I am not sure if the given ans is right

What is the value of the integer n?

(1) n(n + 2) = 15 --> \(n^2+2n-15=0\) --> \((n+5)(n-3)=0\) --> \(n=-5\) or \(n=3\). Not sufficient.

(2) (n + 2)^n = 125 --> as \(n\) is an integer then \((n + 2)^n=125^1=5^3\) (125 can be written as integer^integer only in those two ways), only \(n=3\) works: \((2+3)^3=5^3=125\). Sufficient.

Re: What is the value of the integer n....Exponents [#permalink]
01 Jan 2011, 07:12

but talking about statement 2), cant value of N be 3 or ANY decimal number? Since it is NOT stated in the question that "N is an integer". Correct me if I am wrong.

Re: What is the value of the integer n....Exponents [#permalink]
01 Jan 2011, 07:18

Expert's post

girishkakkar wrote:

but talking about statement 2), cant value of N be 3 or ANY decimal number? Since it is NOT stated in the question that "N is an integer". Correct me if I am wrong.

Question is: What is the value of the integer n? So the stem explicitly says that n is an integer. _________________

Re: What is the value of the integer n? (1) n(n + 2) = 15 (2) (n [#permalink]
04 Jan 2012, 13:14

Agree with Buneul. If n is integer, them then S2 has to be an integer.

1. Clearly insufficient, two roots = -5,3 2. Since n is integer, only n that satisfies this equation is n = 3. Sufficient

+B. _________________

I am the master of my fate. I am the captain of my soul. Please consider giving +1 Kudos if deserved!

DS - If negative answer only, still sufficient. No need to find exact solution. PS - Always look at the answers first CR - Read the question stem first, hunt for conclusion SC - Meaning first, Grammar second RC - Mentally connect paragraphs as you proceed. Short = 2min, Long = 3-4 min

Re: What is the value of the integer n....Exponents [#permalink]
02 Jan 2013, 07:53

Bunuel wrote:

ajit257 wrote:

What is the value of the integer n?

(1) n(n + 2) = 15 (2) (n + 2)^n = 125

What is the value of the integer n?

(1) n(n + 2) = 15 --> \(n^2+2n-15=0\) --> \((n+5)(n-3)=0\) --> \(n=-5\) or \(n=3\). Not sufficient.

Answer: B.

Dear Bunuel,

My fist approach for statement 1 is 15=3X5, so my reasoning is n(n+2)=3X5. Only n=3 match to that equation, so I pick up D. Could you tell me any thing wrong from my reasoning? THX!

Re: What is the value of the integer n....Exponents [#permalink]
03 Jan 2013, 02:13

Expert's post

curtis0063 wrote:

Bunuel wrote:

ajit257 wrote:

What is the value of the integer n?

(1) n(n + 2) = 15 (2) (n + 2)^n = 125

What is the value of the integer n?

(1) n(n + 2) = 15 --> \(n^2+2n-15=0\) --> \((n+5)(n-3)=0\) --> \(n=-5\) or \(n=3\). Not sufficient.

Answer: B.

Dear Bunuel,

My fist approach for statement 1 is 15=3X5, so my reasoning is n(n+2)=3X5. Only n=3 match to that equation, so I pick up D. Could you tell me any thing wrong from my reasoning? THX!

Re: What is the value of the integer n? (1) n(n + 2) = 15 (2) (n [#permalink]
02 Aug 2014, 12:06

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email. _________________

Re: What is the value of the integer n? (1) n(n + 2) = 15 (2) (n [#permalink]
04 Sep 2014, 06:10

curtis0063 wrote:

Bunuel wrote:

ajit257 wrote:

What is the value of the integer n?

(1) n(n + 2) = 15 (2) (n + 2)^n = 125

What is the value of the integer n?

(1) n(n + 2) = 15 --> \(n^2+2n-15=0\) --> \((n+5)(n-3)=0\) --> \(n=-5\) or \(n=3\). Not sufficient.

Answer: B.

Dear Bunuel,

My fist approach for statement 1 is 15=3X5, so my reasoning is n(n+2)=3X5. Only n=3 match to that equation, so I pick up D. Could you tell me any thing wrong from my reasoning? THX!

15 can also be written as -3 * -5... if n= -5 then n(n+2) => -5 * -3= 15 So A cannot provide an unique solution.

Re: What is the value of the integer n? (1) n(n + 2) = 15 (2) (n [#permalink]
21 Sep 2015, 07:12

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email. _________________

Re: What is the value of the integer n? (1) n(n + 2) = 15 (2) (n [#permalink]
22 Sep 2015, 03:52

Expert's post

Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution.

What is the value of the integer n?

(1) n(n + 2) = 15 (2) (n + 2)^n = 125

In the original condition there is 1 variable (n) and thus we need 1 equation to match the number of variable and equation. Since there is 1 each in 1) and 2), D has high probability of being the answer. In case of 1), n^2+2n-15=0, (n+5)(n-3)=0 and thus n=-5,3. The answer is not unique, therefore the condition is not sufficient. In case of 2), n=3, therefore the answer is unique and the condition is sufficient. Therefore the answer is B.

Normally for cases where we need 1 more equation, such as original conditions with 1 variable, or 2 variables and 1 equation, or 3 variables and 2 equations, we have 1 equation each in both 1) and 2). Therefore D has a high chance of being the answer, which is why we attempt to solve the question using 1) and 2) separately. Here, there is 59 % chance that D is the answer, while A or B has 38% chance. There is 3% chance that C or E is the answer for the case. Since D is most likely to be the answer according to DS definition, we solve the question assuming D would be our answer hence using 1) and 2) separately. Obviously there may be cases where the answer is A, B, C or E. _________________

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