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If x>1, what is the value of integer x? 1) There are x [#permalink]
10 Aug 2011, 17:45

00:00

A

B

C

D

E

Difficulty:

65% (hard)

Question Stats:

42% (01:15) correct
58% (00:41) wrong based on 12 sessions

If x>1, what is the value of integer x?

1) There are x unique factors of x. 2) The sum of x and any prime number larger than x is odd.

The MGMAT book explains the answer as 1) S, 2) NS. They say that in order for statement one to be true, every integer between 1 and x, inclusive, must be a factor of x. By testing numbers, this holds true for 1 and 2, but not for 3 and 4. (Page 31 of the number properties guide if anyone cares to look).

Can someone please explain to me why this does not hold true for the numbers 3 and 4. Not sure what I am missing here.

Re: Odds and Evens - DS - Manhattan GMAT [#permalink]
10 Aug 2011, 19:03

1

This post received KUDOS

jgonza8 wrote:

If x>1, what is the value of integer x?

x is an integer greater than 1. What is the value of x ? 1) There are x unique factors of x. This can happen for 2 only if x>1...because 2 has 2 factors - 1,2 All other values of x do not x unique factors - example - 3 has only 1 unique factors (1,3) other than 3 itself because\(3 = 1*3\) 4 has only 2 unique factors (1,2) other than 4 itself because \(4 = 1*2^2\) 5 has only 1 unique factors (1,5) other than 5 itself because \(5 = 1*5\) 6 has only 3 unique factors (1,2,3) other than 6 itself because \(6 = 1*2*3\) So A/D now 2) The sum of x and any prime number larger than x is odd. Let us substitute...x will only be even value because odd - odd = even 2+5 = 7 4+5 = 9 So multiple values...So B is not sufficient. So A will be the OA

The MGMAT book explains the answer as 1) S, 2) NS. They say that in order for statement one to be true, every integer between 1 and x, inclusive, must be a factor of x. By testing numbers, this holds true for 1 and 2, but not for 3 and 4. (Page 31 of the number properties guide if anyone cares to look).

Can someone please explain to me why this does not hold true for the numbers 3 and 4. Not sure what I am missing here. Have explained above.

Re: Odds and Evens - DS - Manhattan GMAT [#permalink]
11 Aug 2011, 21:06

1

This post received KUDOS

the question asks what number is equal to the number of his unique factors. 1 has 1 unique factor so 1 is equal to the number of his unique factors. 2 is equal to the number of his unique factors. 3 has only 2 unique factors , 4 and 5 have 2, 11 only 2 and so on.

so the only numbers equal to their unique fators are 1 and 2 . the condition is that x>1 so only 2 applies.

Affiliations: CFA Institute (CFA Candiate), Canadian Institute of Chartered Accountants (CA Candiate), Chartered Alternative Investments Analysts Association (CAIA Candidate)

Re: Odds and Evens - DS - Manhattan GMAT [#permalink]
12 Aug 2011, 17:11

Sovjet wrote:

Why are you guys not considering the number itself as a factor?

To me:

4 has 3 unique factors (not 2) - 1,2,4 5 has 2 unique factors - 1,5 6 has 4 (not 3) unique factors - 1,2,3,6

It doesn't change the answer or anything, but the number itself is a unique factor. They weren't asking for the prime roots.

Yes - This is correct.....I had missed this...

And why does this not affect the answer ? Because Total number of unique factors for any number = 1 + Total number of unique factors other than the number Adding 1 does not affect the count because we are any way comparing the (actual count - 1) However my solution will be wrong if applied for larger numbers say 12 \(12 = 3 *2^2\) Number of unique factors other than \(1 = (1+1)*(2+1) - 1 = 2*3 - 1 = 5\)

You can also calculate and confirm this : Factors of 12 - 1,2,3,4,6,12 Factors of 12 other than 1 = 2,3,4,6,12 All are unique Total number of such unique factors other than 1 = 5

So - Substitute smartly because this is a DATA SUFFICIENCY and NOT PROBLEM SOLVING question. I am editing my post to correct this .... _________________

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