OG math.

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givemeanid: Master | Next Rank: 500 Posts; Posts: 277; Joined: Sun Jun 17, 2007 2:51 pmFollowed by: 1 member; Location: New York, NY; Thanked: 6 times

by givemeanid » Mon Sep 03, 2007 12:58 pm

'could be' is the key. Question is not asking which value will 'always be' a prime. Check other choices and you will realize there is no value of X for which the other choices will ever be true.

So It Goes

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Source: Beat The GMAT — Problem Solving | Mon Sep 03, 2007 12:58 pm

farooq: Master | Next Rank: 500 Posts; Posts: 140; Joined: Sat Feb 28, 2009 8:51 amFollowed by: 3 members; Location: India; Thanked: 14 times

Re: OG math.

by farooq » Fri Oct 30, 2009 7:54 am

aditya33 wrote:If X is a prime number greater than 2.Which of the following could be a prime number??

1-X^2
2-X/2
3-3X
4-X-4
5-X^2+1..

Ans is:- d (X-4)..But if i choose 13 as a prime number then it could be 13-4=9 which is not a prime numeber.Although 11-4=7 which is a prime no.So,this ans gives us two different answers..I am confused which one should be correct..
Plss help.

If X is a prime number greater than 2.Which of the following could be a prime number??

It means is there any choice that can be a prime no. for X.

Only option D full fill this possibility.

Regards,
Farooq Farooqui.
London. UK

It is your Attitude, not your Aptitude, that determines your Altitude.

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chipbmk: Senior | Next Rank: 100 Posts; Posts: 70; Joined: Mon Oct 05, 2009 9:33 am

Re: OG math.

by chipbmk » Fri Oct 30, 2009 10:02 am

aditya33 wrote:If X is a prime number greater than 2.Which of the following could be a prime number??

1-X^2
2-X/2
3-3X
4-X-4
5-X^2+1..

Ans is:- d (X-4)..But if i choose 13 as a prime number then it could be 13-4=9 which is not a prime numeber.Although 11-4=7 which is a prime no.So,this ans gives us two different answers..I am confused which one should be correct..
Plss help.

Although the other posters seem to have answered your question, I thought I could provide slightly more detail.

1- x^2 can't be a prime because it will be divisible by 1, x^2, and x.
2- x/2 can't be prime because x is odd and an odd divided by 2 is not an integer
3- 3x can't be prime because it will have 1, x, and 3 as factors.
4- you proved this one yourself
5- x^2+1 can't be prime because if x is odd (as question says it is) than once you add 1 it will be an even. No even greater than 2 is a prime.

Hope that was helpful.

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