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GMAT Club Legend  V
Joined: 11 Sep 2015
Posts: 4959
GMAT 1: 770 Q49 V46
If x and y are integers, and N = (x + y)(2x – y)(x + 2y  [#permalink]

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Top Contributor
7 00:00

Difficulty:   85% (hard)

Question Stats: 53% (02:40) correct 47% (02:28) wrong based on 182 sessions

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If x and y are integers, and N = (x + y)(2x – y)(x + 2y + 1), which of the following is true?

A) If N is even, then x must be even
B) If N is odd, then x must be odd
C) If N is even, then x and y must both be odd
D) If N is odd, then y must be odd
E) If N is even, then y must be even

*kudos for all correct solutions

_________________
GMAT Club Legend  V
Joined: 11 Sep 2015
Posts: 4959
GMAT 1: 770 Q49 V46
If x and y are integers, and N = (x + y)(2x – y)(x + 2y  [#permalink]

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Top Contributor
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GMATPrepNow wrote:
If x and y are integers, and N = (x + y)(2x – y)(x + 2y + 1), which of the following is true?

A) If N is even, then x must be even
B) If N is odd, then x must be odd
C) If N is even, then x and y must both be odd
D) If N is odd, then y must be odd
E) If N is even, then y must be even

*kudos for all correct solutions

Given the various answer choices, it might be useful to take a systematic approach and first look at all 4 possible cases.

Case i) x is EVEN and y is EVEN
Case ii) x is EVEN and y is ODD
Case iii) x is ODD and y is EVEN
Case iv) x is ODD and y is ODD

For each case, we can EITHER apply the rules for even and odd numbers (e.g., EVEN + ODD = ODD) OR we can just plug in some easy numbers.
I’ll go the plugging in route.

For even numbers, I’ll plug in 0, and for odd numbers I’ll plug in 1. Here’s what we get:
Case i) x = 0, y = 0, so N = (0)(0)(1) = 0. Result: N is EVEN
Case ii) x = 0, y = 1, so N = (1)(-1)(3) = -3. Result: N is ODD
Case iii) x = 1, y = 0, so N = (1)(2)(2) = 4. Result: N is EVEN
Case iv) x = 1, y = 1, so N = (2)(1)(4) = 8. Result: N is EVEN

So, we have:
Case i) x is EVEN and y is EVEN. N is EVEN
Case ii) x is EVEN and y is ODD. N is ODD
Case iii) x is ODD and y is EVEN. N is EVEN
Case iv) x is ODD and y is ODD. N is EVEN

A) If N is even, then x must be even. FALSE. Cases iii and iv refute this statement.
B) If N is odd, then x must be odd. FALSE. Case ii refutes this statement.
C) If N is even, then x and y must both be odd. FALSE. Cases i and iii refute this statement.
D) If N is odd, then y must be odd. TRUE. See case ii
E) If N is even, then y must be even. FALSE. Case iv refutes this statement.

_________________

Originally posted by BrentGMATPrepNow on 13 Apr 2017, 09:53.
Last edited by BrentGMATPrepNow on 02 May 2020, 09:26, edited 1 time in total.
##### General Discussion
Manager  G
Joined: 28 Jul 2016
Posts: 128
Re: If x and y are integers, and N = (x + y)(2x – y)(x + 2y  [#permalink]

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2
1
(x+y)*(2x-y)*(x+2y+1)

Result of multiplication is Odd in case all multiplicands are Odd

1) x+y = Odd (x or y should be Even, and y or x should be Odd)
2) 2x-y = Odd (2x Even, then y must be Odd)
3) x+2y-1 = Odd (2y - Even, 1 Odd, then x must be Even)

RSM Erasmus Moderator V
Joined: 26 Mar 2013
Posts: 2473
Concentration: Operations, Strategy
Schools: Erasmus
Re: If x and y are integers, and N = (x + y)(2x – y)(x + 2y  [#permalink]

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2
1
GMATPrepNow wrote:
If x and y are integers, and N = (x + y)(2x – y)(x + 2y + 1), which of the following is true?

A) If N is even, then x must be even
B) If N is odd, then x must be odd
C) If N is even, then x and y must both be odd
D) If N is odd, then y must be odd
E) If N is even, then y must be even

Let's simplify N first.

2x=Even & 2y+1=Even + ODD=ODD

Hence.... N= (x + y)(E – y)(x + O)

Let see when is either Even or Odd..Here I will borrow Brent's table above Case i) x is EVEN and y is EVEN.............N= (E+E) (E-E) (E+O)= EVEN

Case ii) x is EVEN and y is ODD..............N = (E+O) (E-O) (E+O)= ODD

Case iii) x is ODD and y is EVEN.............N = (O+E) (E-E) (O+O) = Even

Case iv) x is ODD and y is ODD............. N = (O+O) (E-O) (O+O) = Even

So we have one 3 cases with Even and 1 case with ODD.. So I will focus my effort when N is ODD....Eliminate A, C & E

To be ODD, y has to be ODD

Non-Human User Joined: 09 Sep 2013
Posts: 15408
Re: If x and y are integers, and N = (x + y)(2x – y)(x + 2y  [#permalink]

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_________________ Re: If x and y are integers, and N = (x + y)(2x – y)(x + 2y   [#permalink] 25 May 2020, 02:52

# If x and y are integers, and N = (x + y)(2x – y)(x + 2y  