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If x, y and z are integers and xy + z is an odd integer, is

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If x, y and z are integers and xy + z is an odd integer, is [#permalink] New post 28 Jul 2009, 21:25
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If x, y and z are integers and xy + z is an odd integer, is x an even integer?

(1) xy + xz is an even integer
(2) y + xz is an odd integer
[Reveal] Spoiler: OA

Last edited by Bunuel on 07 Mar 2013, 01:10, edited 1 time in total.
Edited the question and added the OA.
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Re: DS - Is x even? [#permalink] New post 28 Jul 2009, 22:14
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IMO A

1. xy + xz is an even integer - SUFFICIENT
Given:
xy + z is odd ...(i)
xy + xz is even ...(ii)

subtracting (ii) from (i)
we get xz - z, which should be odd (* since odd - even = odd)
=> z(x-1) is odd
=> both z and (x-1) is odd
=> since (x-1) is odd, x must be even.

2. y + xz is an odd integer -INSUFFICIENT
Given:
xy + z is odd ...(i)
y + xz is odd ...(ii)

subtracting (ii) from (i)
we get xy + z - y - xz
= (x-1)(y-z) , which should be even
=> either (x-1) is even or (y-z) is even ....insufficient to determine
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Re: GMAT Prep...How much time did u take to solve this one ?? [#permalink] New post 26 Sep 2009, 21:52
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You can look at Statement 1 conceptually: when we add z to xy, we get something odd. However, when we add xz to xy, we get something even. So certainly one of z or xz is odd, the other even. Now if xz is different from z, then multiplying by x must have changed z, and that could only happen if x is even and z odd. That's a bit tricky to explain, but I hope that's clear.

Or you can proceed algebraically - notice the similarity between the expression in the question and in Statement 1. We know that xy + xz is even, and xy + z is odd. When you subtract this second expression from the first, you're subtracting an odd from an even, so must get an odd: xy + xz - (xy + z) = xz - z = z(x-1) is odd. Since this is a product, z must be odd, and x-1 must be odd, so x is even. Sufficient.

For Statement 2, all the letters could be odd, so not sufficient.
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Re: GMAT Prep...How much time did u take to solve this one ?? [#permalink] New post 27 Sep 2009, 08:58
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IanStewart wrote:
You can look at Statement 1 conceptually: when we add z to xy, we get something odd. However, when we add xz to xy, we get something even. So certainly one of z or xz is odd, the other even. Now if xz is different from z, then multiplying by x must have changed z, and that could only happen if x is even and z odd. That's a bit tricky to explain, but I hope that's clear.

Or you can proceed algebraically - notice the similarity between the expression in the question and in Statement 1. We know that xy + xz is even, and xy + z is odd. When you subtract this second expression from the first, you're subtracting an odd from an even, so must get an odd: xy + xz - (xy + z) = xz - z = z(x-1) is odd. Since this is a product, z must be odd, and x-1 must be odd, so x is even. Sufficient.

For Statement 2, all the letters could be odd, so not sufficient.


Fr St2,
y+xz odd
xy+z odd
=> y+zx+xy+z even
=> y(x+1)+z(x+1) even
=> (y+z)(x+1) event
x+1 can be odd or even means that x can be even or odd, insuff
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Re: GMAT Prep...How much time did u take to solve this one ?? [#permalink] New post 28 Sep 2009, 04:34
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It took me 15 seconds.

1) sufficient (x is even)
2) insufficient (try even y, odd x, odd z; even x, odd y, z)

A
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Re: GMAT Prep...How much time did u take to solve this one ?? [#permalink] New post 29 Sep 2009, 18:26
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Took me less than a min

if XY+Z is odd
these are the possiblities

X Y Z

E E O
O E O
E O O
O O E

so substituting these values in Option A gives the solution write away as Even.
in B we have 2 diff results so

Answer is A
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Re: If x, y and z are integers and xy + z is an odd integer, is [#permalink] New post 31 Jan 2014, 08:31
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Aleehsgonji wrote:
If x, y and z are integers and xy + z is an odd integer, is x an even integer?

(1) xy + xz is an even integer
(2) y + xz is an odd integer



Odd/Even questions can be usually solved quite easily if one tries some operations with the statements

We want to know if x is even integer

We are given that xy+z is odd

Statement 1

xq + xz is even

Subtracting

z(x+1) is odd

Therefore, x+1 should be odd and x should be even

Sufficient

Statement 2

Not sufficient

Answer is A

Just my 2c

Cheers
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Re: If x, y and z are integers and xy + z is an odd integer, is [#permalink] New post 19 Mar 2014, 18:41
Given condition:
xy + z = odd
implies either xy = odd (x =odd and y = odd) and z = even or xy = even (x or y can be odd and even respectively and vice versa) and z = odd

condition 1:

xy + xz = even; Implies x(y+z) = even which again implies the following:

i) x even and y+z = odd - where again y or z can be odd and even respectively and vice versa
ii) x odd and y +z = even - where again y and z has to be both odd or both even

inconclusive

condition 2:

y + xz = odd

again inconclusive
1 + 2:
Add xy + z + y + xz = odd + odd
implies: (x + 1)(y+z) = even
and x (y+z) is also even according to 2.. so y + z = even <y and z both even or y + z both odd>, x can be odd or even
but by 1 xy + z = odd which means y and z both odd, so x is even.

C is the answer
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Re: If x, y and z are integers and xy + z is an odd integer, is [#permalink] New post 20 Mar 2014, 05:09
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Mountain14 wrote:
jlgdr wrote:
Aleehsgonji wrote:
If x, y and z are integers and xy + z is an odd integer, is x an even integer?

(1) xy + xz is an even integer
(2) y + xz is an odd integer



Odd/Even questions can be usually solved quite easily if one tries some operations with the statements

We want to know if x is even integer

We are given that xy+z is odd

Statement 1

xq + xz is even

Subtracting

z(x+1) is odd

Therefore, x+1 should be odd and x should be even


Sufficient

Statement 2

Not sufficient

Answer is A

Just my 2c

Cheers
J



I am not clear with the red part.


When you subtract \(xy + z=odd\) from \(xy+xz=even\) you'll get: \(xz-z=even-odd=odd\) --> \(z(x-1)=odd\). For the product of two integers to be odd, both of them must be odd --> \(z\) and \(x-1\) are odd. If \(x-1=odd\), then x must be even: \(x-1=x-odd=odd\) --> \(x=odd+odd=even\).

Hope it's clear.
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Re: If x, y and z are integers and xy + z is an odd integer, is [#permalink] New post 21 Mar 2014, 19:46
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Odd(O) Even (E)
given:
x,y,z integers
xy+z=O
so only the following scenarios can fulfill the constraints
a) EO+O
b) EE+O
c) OE+O
d) OO+E

question:
x=E?

1) x(y+z)=E
i. (E)(O+O) --> fits scenario a -->yes, x can be even
ii. (O)(E+E) --> n/a - doesn't fit any scenarios
iii. (O)(O+O) --> n/a - doesn't fit any scenarios

stop testing, x can't be odd, sufficient

2) y+xz = O
i. E+(O)(O) --> fits scenario a -->yes, x can be even
ii. O+(E)(E) --> n/a - doesn't fit any scenarios
iii. O+(O)(E) --> fits scenario d -->no, x can be odd

stop testing, x can be either even or odd

insufficient

A
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Re: GMAT Prep...How much time did u take to solve this one ?? [#permalink] New post 30 Sep 2014, 04:10
nikhilpoddar wrote:
Please solve...


took me 4 seconds, but I got it wrong :(
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Re: If x, y and z are integers and xy + z is an odd integer, is [#permalink] New post 23 Feb 2015, 05:56
Amazing explanation as usual Bunuel. May I know how did you validate statement (2) !

[quote="Bunuel"][quote="Mountain14"][quote="jlgdr"][quote="Aleehsgonji"]If x, y and z are integers and xy + z is an odd integer, is x an even integer?

(1) xy + xz is an even integer
(2) y + xz is an odd integer
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Re: If x, y and z are integers and xy + z is an odd integer, is [#permalink] New post 23 Feb 2015, 07:12
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Re: If x, y and z are integers and xy + z is an odd integer, is   [#permalink] 23 Feb 2015, 07:12
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