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# M04-31

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Math Expert
Joined: 02 Sep 2009
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16 Sep 2014, 00:23
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Difficulty:

35% (medium)

Question Stats:

69% (01:13) correct 31% (01:23) wrong based on 166 sessions

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A perfect number is defined as one for which the sum of all the unique factors less the number itself is equal to the number. For instance, 6 is a perfect number, because the factors of 6 (apart from 6 itself) are 1, 2 and 3, and $$1 + 2 + 3 = 6$$. Which of the following is also a perfect number?

A. 12
B. 20
C. 28
D. 48
E. 60
[Reveal] Spoiler: OA

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Joined: 02 Sep 2009
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16 Sep 2014, 00:23
Official Solution:

A perfect number is defined as one for which the sum of all the unique factors less the number itself is equal to the number. For instance, 6 is a perfect number, because the factors of 6 (apart from 6 itself) are 1, 2 and 3, and $$1 + 2 + 3 = 6$$. Which of the following is also a perfect number?

A. 12
B. 20
C. 28
D. 48
E. 60

$$28 = 1+2+4+7+14$$

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Joined: 27 Jun 2014
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04 Oct 2014, 10:57
How we have calculated Factor of 28 ? 4 and 14 are not prime .. is it correct to show factor as non prime number ?
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04 Oct 2014, 11:11
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rahulgmat2014 wrote:
How we have calculated Factor of 28 ? 4 and 14 are not prime .. is it correct to show factor as non prime number ?

Factor of an integer is not necessarily a prime number.

A divisor of an integer $$n$$, also called a factor of $$n$$, is an integer which evenly divides $$n$$ without leaving a remainder. In general, it is said $$m$$ is a factor of $$n$$, for non-zero integers $$m$$ and $$n$$, if there exists an integer $$k$$ such that $$n = km$$.

So, the factors of 28 are 1, 2, 4, 7, 14, and 28.

Theory on Number Properties: math-number-theory-88376.html
Divisibility tips: divisibility-multiples-factors-tips-and-hints-174998.html?hilit=divisibility%20tips

DS Divisibility/Multiples/Factors questions to practice: search.php?search_id=tag&tag_id=354
PS Divisibility/Multiples/Factors questions to practice: search.php?search_id=tag&tag_id=185[/textarea]

Hope it helps.
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Joined: 11 Mar 2014
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16 Nov 2014, 16:55
So when it says "unique factors' - that does not equal prime?
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17 Nov 2014, 02:01
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phferr1984 wrote:
So when it says "unique factors' - that does not equal prime?

No, Unique factors = different factors.
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Joined: 25 Jan 2013
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Concentration: General Management, Entrepreneurship

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09 Sep 2016, 07:23
Bunuel wrote:
Official Solution:

$$28 = 1+2+4+7+14$$

Why 12 is incorrect 1+2+3+6 = 12
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09 Sep 2016, 08:04
vs224 wrote:
Bunuel wrote:
Official Solution:

$$28 = 1+2+4+7+14$$

Why 12 is incorrect 1+2+3+6 = 12

4 is also a factor of 12.
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Joined: 03 Sep 2017
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03 Mar 2018, 15:56
Bunuel, any way to narrow down some options, so it wont be 100% try and error?
Re: M04-31   [#permalink] 03 Mar 2018, 15:56
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# M04-31

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