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GRE Math Challenge #25-the least positive integer formed

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GRE Math Challenge #25-the least positive integer formed [#permalink] New post 08 Sep 2014, 01:21
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Question Stats:

37% (00:51) correct 62% (00:27) wrong based on 8 sessions
column A: the least positive integer formed by four different prime factors all of which are greater than 2

column B : 1155

A)The quantity in Column A is greater
B)The quantity in Column B is greater
C)Both are Equal.
D)The relationship cannot be determined from the information given
[Reveal] Spoiler: OA

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Re: GRE Math Challenge #25-the least positive integer formed [#permalink] New post 07 Mar 2018, 00:01
The stem asks us the least positive integer formed by 4 DIFFERENT prime factors>2.

Then, we can think that the 4 primes are 3,5, 7,11.

The products of these primes are 1155. ANS=C
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Re: GRE Math Challenge #25-the least positive integer formed [#permalink] New post 07 Mar 2018, 09:42
but why is it not specified in the question what does it want, if not specified to multiply the primes, i can just add them which is<1155, then the ans must be D, cannot be determined
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Re: GRE Math Challenge #25-the least positive integer formed [#permalink] New post 07 Mar 2018, 15:53
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viki69 wrote:
but why is it not specified in the question what does it want, if not specified to multiply the primes, i can just add them which is<1155, then the ans must be D, cannot be determined


A number formed by adding prime number do not have the same prime as factors.

Example 2 and 3 are prime numbers; 6 or 2 * 3 has prime factors 2 and 3.
But 2+ 3= 5 which doe snot have prime factors as 2 and/or 3.
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Re: GRE Math Challenge #25-the least positive integer formed [#permalink] New post 08 Mar 2018, 00:22
sandy wrote:
viki69 wrote:
but why is it not specified in the question what does it want, if not specified to multiply the primes, i can just add them which is<1155, then the ans must be D, cannot be determined


A number formed by adding prime number do not have the same prime as factors.

Example 2 and 3 are prime numbers; 6 or 2 * 3 has prime factors 2 and 3.
But 2+ 3= 5 which doe snot have prime factors as 2 and/or 3.


So u mean to say that whenever factors are talked about, we have to consider multiplication and noting else, but there are other cases too, for example, 3+7=10=2*5 all are prime of 10 :(
I apologise if you get frustated because of my reasoning, but please help me understand this concept. Thanks in advance.
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Re: GRE Math Challenge #25-the least positive integer formed [#permalink] New post 08 Mar 2018, 15:02
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viki69 wrote:
but why is it not specified in the question what does it want, if not specified to multiply the primes, I can just add them which is<1155, then the answer must be D, cannot be determined


When you perform the prime factorization of a number, for instance, 10, of which the prime numbers are 2 and 5, then you to BACK to the previous number you must do a multiplication.

So, if the question asks you for a number composed of prime numbers > 2 the number as result must be the product of these numbers NOT the addition.

Hope now is clear
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Re: GRE Math Challenge #25-the least positive integer formed   [#permalink] 08 Mar 2018, 15:02
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