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x and y are positive integers. If xy + x is odd, then which of the
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Bunuel
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x and y are positive integers. If xy + x is odd, then which of the [#permalink] New post  19 Aug 2016, 02:07
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x and y are positive integers. If xy + x is odd, then which of the following must be even?

A. x
B. y
C. x + y
D. xy − x
E. x^2 − y
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Attempt only this question Same topic and source Same topic and difficulty Same source and difficulty Random Chronological Reverse chronological Learn more
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msk0657
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x and y are positive integers. If xy + x is odd, then which of the [#permalink] New post  19 Aug 2016, 03:15
Bunuel wrote:
x and y are positive integers. If xy + x is odd, then which of the following must be even?

A. x
B. y
C. x + y
D. xy − x
E. x^2 − y


x y xy + x
1 2 3 odd
2 1 4 Even case
2 3 5 Even
5 2 15 odd
3 3 12 even

IMO option B is correct.

OA please..let me know if I am missing anything...
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BrentGMATPrepNow
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x and y are positive integers. If xy + x is odd, then which of the [#permalink] New post  Updated on: 02 May 2020, 09:20
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Bunuel wrote:
x and y are positive integers. If xy + x is odd, then which of the following must be even?

A. x
B. y
C. x + y
D. xy − x
E. x^2 − y


Some important rules regarding the products of evens and odds:
1. (ODD)(ODD) = ODD
2. (ODD)(EVEN) = EVEN
3. (EVEN)(EVEN) = EVEN
4. ODD +/- ODD = EVEN
5. ODD +/- EVEN = ODD
6. EVEN +/- EVEN = EVEN
As we can see from #1, IF the product of two integers is ODD, then the two integers must both be odd.

Given: xy + x is ODD
If we FACTOR xy + x, we get x(y + 1)
So, we're told that x(y + 1) is ODD
From rule #1 above, we can conclude that x is ODD and (y + 1) is ODD
If (y + 1) is odd, then we can be certain that y is EVEN

Now check the answer choices:
A. x = ODD
B. y = EVEN (done!!)
C. x + y = ODD + EVEN = ODD
D. xy − x = (ODD)(EVEN) - ODD = EVEN - ODD = ODD
E. x^2 − y = (ODD)(ODD) - EVEN = ODD - EVEN = ODD

Answer:
Show Spoiler
B

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Originally posted by BrentGMATPrepNow on 19 Aug 2016, 05:05.
Last edited by BrentGMATPrepNow on 02 May 2020, 09:20, edited 1 time in total.
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kuksbug
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Re: x and y are positive integers. If xy + x is odd, then which of the [#permalink] New post  20 Aug 2016, 01:04
xy+ x= odd

Two possibilities:
(1) even + odd = odd
(2) odd + even= odd
In this case, x can not be even as it will make xy also even .
Therefore, (1) possibility present => y must be even.

Answer: B
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hsambiguity
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Re: x and y are positive integers. If xy + x is odd, then which of the [#permalink] New post  22 Aug 2016, 00:33
Given that xy + x is odd, which means x(y+1) is odd.
Product of two nos. will be odd only if both the nos. are odd.
Hence x is odd, y+1 is also odd.
y+1 is odd means y is even. Answer will be B.
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RR88
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Re: x and y are positive integers. If xy + x is odd, then which of the [#permalink] New post  17 Apr 2017, 02:51
Option B

x & y are Positive Integers. Odd: O, Even: E & Fraction: F.
xy + x = O
i.e., x = O & y = E
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JeffTargetTestPrep
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Re: x and y are positive integers. If xy + x is odd, then which of the [#permalink] New post  20 Apr 2017, 15:56
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Bunuel wrote:
x and y are positive integers. If xy + x is odd, then which of the following must be even?

A. x
B. y
C. x + y
D. xy − x
E. x^2 − y


We are given that x and y are positive integers and that xy + x is odd, or x(y + 1) is odd, which means that x is odd and y is even. Thus, y MUST be even.

Answer: B
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Re: x and y are positive integers. If xy + x is odd, then which of the [#permalink] New post  26 Apr 2021, 09:59
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Re: x and y are positive integers. If xy + x is odd, then which of the [#permalink]
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