X & Y as integers

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relaxin99: Senior | Next Rank: 100 Posts; Posts: 73; Joined: Tue Sep 16, 2008 4:33 pm; Thanked: 2 times

by relaxin99 » Thu Feb 12, 2009 2:37 pm

If x & y are integers, is the value of x(y + 1) even?

(1) x & y are prime numbers
(2) y > 7

The answer is C, can someone please explain why

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Source: Beat The GMAT — Data Sufficiency | Thu Feb 12, 2009 2:37 pm

cramya: Legendary Member; Posts: 2467; Joined: Thu Aug 28, 2008 6:14 pmFollowed by: 11 members; Thanked: 331 times

by cramya » Thu Feb 12, 2009 3:14 pm

X(Y+1) = XY+X

Stmt I

x and y are prime numbers.

If x and y are both 2 then we get an even result
If x is even, y is odd we get an odd result
INSUFF

Stmt II

y>7

No info about x
Could be even or odd

Together

Since y>7 y is always odd

When x is odd

xy+x = odd*odd + odd
=even

When x is even
xy+x = even*odd+even
= even

SUFF

Choose C

Regards,
CR

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logitech: Legendary Member; Posts: 2134; Joined: Mon Oct 20, 2008 11:26 pmFollowed by: 25 members; Thanked: 237 times; GMAT Score:730

Re: X & Y as integers

by logitech » Thu Feb 12, 2009 6:35 pm

All primes , but 2 , are ODD

ODD(ODD+1) = EVEN

But if one of them is 2

2(ODD+1) = EVEN
ODD(2+1) = ODD
2(2+1) = EVEN

so if we know that Y is not 2 we can solve the problem.
'
Choose C

LGTCH
---------------------
"DON'T LET ANYONE STEAL YOUR DREAM!"

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x2suresh: Master | Next Rank: 500 Posts; Posts: 258; Joined: Thu Aug 07, 2008 5:32 am; Thanked: 16 times

Re: X & Y as integers

by x2suresh » Thu Feb 12, 2009 7:45 pm

agree with C

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