PR-4: Q - 26

This topic has expert replies
Legendary Member
Posts: 645
Joined: Wed Sep 05, 2007 4:37 am
Location: India
Thanked: 34 times
Followed by:5 members

PR-4: Q - 26

by camitava » Mon Jun 23, 2008 10:25 pm
Guys help me in this question -

If f and g are integers, is fg + 2 prime?
1. f is odd.
2. g is even.

I will provide the OA later. What do u think the answer will be and why?
Correct me If I am wrong


Regards,

Amitava

User avatar
Legendary Member
Posts: 543
Joined: Fri Jan 18, 2008 1:01 am
Thanked: 43 times
GMAT Score:580

by codesnooker » Tue Jun 24, 2008 1:41 am
Answer should be (E).

1. f is odd. IN SUFFICIENT, as we don't have any information regarding g.
2. g is even. IN SUFFICIENT, as we don't have any information regarding f.

Lets take both statement together.

fg will be even. As odd X even = even.

Again fg + 2 = even integer (because even + even = even)

However fg + 2 = prime number only when fg = 0 (because 2 is the only even prime number and we have already prove that fg + 2 = even integer).

So any how we don't the value of fg, as it could be anything like -1, 0, 1, 2 etc. So again both (1) and (2) are INSUFFICIENT.

Hence IMO (E)

Hope this helps...

Legendary Member
Posts: 645
Joined: Wed Sep 05, 2007 4:37 am
Location: India
Thanked: 34 times
Followed by:5 members

by camitava » Tue Jun 24, 2008 2:04 am
codesnooker wrote:Answer should be (E).

1. f is odd. IN SUFFICIENT, as we don't have any information regarding g.
2. g is even. IN SUFFICIENT, as we don't have any information regarding f.

Lets take both statement together.

fg will be even. As odd X even = even.

Again fg + 2 = even integer (because even + even = even)

However fg + 2 = prime number only when fg = 0 (because 2 is the only even prime number and we have already prove that fg + 2 = even integer).

So any how we don't the value of fg, as it could be anything like -1, 0, 1, 2 etc. So again both (1) and (2) are INSUFFICIENT.

Hence IMO (E)

Hope this helps...

Nice explanation codesnooker! I agree with u. But if u take this approach -Taking both stmts in account, fg will be even and thus fg + 2 even.
Again as stmt-1and 2 are saying that f and g are odd and even integers, none of f anf g can be zero.
So fg + 2 can not be equal to 2 in any sense!
So fg + 2 will be even other than 2. So fg + 2 can not be prime. So taking both the stmts, we can conclude that fg + 2 can never be prime. So I can go for C.
- Can u pls point out where I am wrong?
Correct me If I am wrong


Regards,

Amitava

User avatar
Legendary Member
Posts: 543
Joined: Fri Jan 18, 2008 1:01 am
Thanked: 43 times
GMAT Score:580

by codesnooker » Tue Jun 24, 2008 2:31 am
camitava wrote: Again as stmt-1and 2 are saying that f and g are odd and even integers, none of f anf g can be zero.
Zero is even integer. I think you are confused with this point. So fg could be zero also. Hence Fg +2 = 2 (if fg = 0). Hence its gives us a prime number. However if fg = 2 or any other even number, then fg + 2 will gives us a non-prime number.

Check the link if you still have any doubt:- https://en.wikipedia.org/wiki/Evenness_of_zero

Legendary Member
Posts: 645
Joined: Wed Sep 05, 2007 4:37 am
Location: India
Thanked: 34 times
Followed by:5 members

by camitava » Tue Jun 24, 2008 3:20 am
Codesnooker, Thanks a lot! Yop! I was confused about the fact Whether zero is an even number or not. If zeros comes as an even number, obviously the best option to chose is E.
Correct me If I am wrong


Regards,

Amitava

Senior | Next Rank: 100 Posts
Posts: 60
Joined: Thu Jun 12, 2008 5:38 pm
Thanked: 2 times

by lvincy » Wed Jun 25, 2008 1:20 pm
Zero is even.
Ans E


Thanks,
Vikas