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# If numbers N and K are added to set X{2,8,10,12}, its mean w

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Joined: 31 Oct 2010
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GMAT 1: 710 Q48 V40
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If numbers N and K are added to set X{2,8,10,12}, its mean w  [#permalink]

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09 Jan 2011, 09:43
3
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15% (low)

Question Stats:

82% (01:20) correct 18% (01:15) wrong based on 232 sessions

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If numbers N and K are added to set X{2,8,10,12}, its mean will increase by 25%. What is the value of $$N^2+2NK+K^2$$ ?

A. 28
B. 32
C. 64
D. 784
E. 3600

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Joined: 02 Sep 2009
Posts: 50543
Re: Numbers added to a set  [#permalink]

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09 Jan 2011, 10:19
4
3
gmatpapa wrote:
If numbers N and K are added to set X{2,8,10,12}, its mean will increase by 25%. What is the value of $$N^2+2NK+K^2$$ ?

A. 28
B. 32
C. 64
D. 784
E. 3600

Mean of {2,8,10,12} is $$mean=\frac{2+8+10+12}{4}=\frac{32}{4}=8$$ --> new mean thus should equal to $$8*1.25=10$$, so $$\frac{2+8+10+12+n+k}{6}=10$$ (note that now we have the set of 6 terms not 4) --> $$n+k=60-32=28$$ --> $$n^2+2nk+k^2=(n+k)^2=28^2=784$$.

Answer: D.
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Joined: 05 Apr 2013
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Re: If numbers N and K are added to set X{2,8,10,12}, its mean w  [#permalink]

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19 Sep 2013, 21:47
If numbers N and K are added to set X{2,8,10,12}, its mean will increase by 25%. What is the value of N^2+2NK+K^2 ?

A. 28
B. 32
C. 64
D. 784
E. 3600

Before adding N & K, mean of X = 32/4 = 8.
After adding N & K, the new mean of X = 125/100*8 = 10.
i,e (32 + N+K )/6 = 10 => N+K = 60-32 = 28

The question is find the value of N^2+2NK+K^2 => (N+K)^2 = 28^2 = 784.
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Re: If numbers N and K are added to set X{2,8,10,12}, its mean w  [#permalink]

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19 Sep 2013, 23:22
2
gmatpapa wrote:
If numbers N and K are added to set X{2,8,10,12}, its mean will increase by 25%. What is the value of $$N^2+2NK+K^2$$ ?

A. 28
B. 32
C. 64
D. 784
E. 3600

You can also use the conceptual and deviation based methods to solve this.

Mean of set X{2,8,10,12} = 8 (Note that if we assume 8 to be the mean, 2 is 6 less than 8 and 10 and 12 are together 6 more than 8. Hence 8 must be the actual mean - the deviation approach)

When we add two numbers, the mean increases by 25% i.e. by 2. Had the two numbers been both 8, the mean would have stayed 8. But they have 2 extra for each of the 6 numbers so they must be a total of 8 + 8 + 6*2 = 28
(N + K)^2 = 28^2 = 784

For more on these, check:
http://www.veritasprep.com/blog/2012/04 ... etic-mean/
http://www.veritasprep.com/blog/2012/05 ... eviations/
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Intern
Joined: 10 Aug 2014
Posts: 3
Re: If numbers N and K are added to set X{2,8,10,12}, its mean w  [#permalink]

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08 Sep 2014, 22:09
Bunuel wrote:
gmatpapa wrote:
If numbers N and K are added to set X{2,8,10,12}, its mean will increase by 25%. What is the value of $$N^2+2NK+K^2$$ ?

A. 28
B. 32
C. 64
D. 784
E. 3600

Mean of {2,8,10,12} is $$mean=\frac{2+8+10+12}{4}=\frac{32}{4}=8$$ --> new mean thus should equal to $$8*1.25=10$$, so $$\frac{2+8+10+12+n+k}{6}=10$$ (note that now we have the set of 6 terms not 4) --> $$n+k=60-32=28$$ --> $$n^2+2nk+k^2=(n+k)^2=28^2=784$$.

Answer: D.

Magician!!! Why do my eyes escape these little details like the a+b whole square formula being expanded in the question ://
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Posts: 8737
Re: If numbers N and K are added to set X{2,8,10,12}, its mean w  [#permalink]

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28 Oct 2018, 21:37
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Re: If numbers N and K are added to set X{2,8,10,12}, its mean w &nbs [#permalink] 28 Oct 2018, 21:37
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# If numbers N and K are added to set X{2,8,10,12}, its mean w

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