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shrey2287: Senior | Next Rank: 100 Posts; Posts: 33; Joined: Thu Apr 15, 2010 9:36 pm; Thanked: 1 times

by shrey2287 » Sun Oct 16, 2011 3:54 am

If x and y are integers.Is x(y+1) even.

1. x and y are prime nos
2. y > 7

ans : C.. (Both Stmts are required)

Plz explain..

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Source: Beat The GMAT — Data Sufficiency | Sun Oct 16, 2011 3:54 am

GmatKiss: Legendary Member; Posts: 2789; Joined: Tue Jul 26, 2011 12:19 amFollowed by: 43 members; Location: Chennai, India; Thanked: 206 times; GMAT Score:640

by GmatKiss » Sun Oct 16, 2011 6:40 am

shrey2287 wrote:If x and y are integers.Is x(y+1) even.

1. x and y are prime nos
2. y > 7

ans : C.. (Both Stmts are required)

Plz explain..

From, 1.
x and y can be any of (2,3,5,7,11,13,17,19...)

lets sub: x(y+1)

x=2; y=5
2(6) =12 - even
x=3; y=2
3(3) = 9 - odd

NOT SUFFICIENT

2. is clearly not sufficient, (we dont know what is X)

NOT SUFFICIENT

1+2 combined,

x can be (2,3,5,7,11,13,17,18....)
y can be (11,13,17,19...)

If you sub, you will know that x(y+1) is always even.

SUFFICIENT.

IMO:C

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tpr-becky: GMAT Instructor; Posts: 509; Joined: Wed Apr 21, 2010 1:08 pmFollowed by: 85 members; Location: Irvine, CA; Thanked: 199 times; GMAT Score:750

by tpr-becky » Mon Oct 17, 2011 4:00 pm

This questin tests the concept of even/odd when two numbers are multiplied together if one of them is even then the product is even.

Therefore we need to know if one of x or (y+1) is even - thus the question is "is x even or is y odd?" if we know the answer to either of these then we can solve.

1) if they are both prime then they could be 2, 3, 5, ... - we don't know for sure whether x is even and we don't know for sure that y odd therefore this is insufficient.

2) y>7 - this tells us nothing about x and y can be even or odd so this is insufficient.

if we know both then we know that y is a prime greater than 7 - the only primes greater than 7 are odd numbers. Therefore we know for certain that y is odd and can determine that the product from the stem will be even. That means the correct answer is C.

Becky
Master GMAT Instructor
The Princeton Review
Irvine, CA

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