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If y and z are integers

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candace_washington1980
 Post subject: If y and z are integers
 Post Posted: Thu Aug 06, 2009 7:01 am 
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Posts: 17
Gmat prep data sufficiency question

If y and z are integers, is y(z + 1) odd?

1. y is odd
2. z is even

I got C which is correct, but I guessed. Please explain why it is C. Thanks.
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furtadovinod
 Post subject: Re: If y and z are integers
 Post Posted: Thu Aug 06, 2009 11:02 am 
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Posts: 28
candace_washington1980 wrote:
Gmat prep data sufficiency question

If y and z are integers, is y(z + 1) odd?

1. y is odd
2. z is even

I got C which is correct, but I guessed. Please explain why it is C. Thanks.


The multiplication of two integers is odd only when both the integers are odd. In this case to prove that y(z+1) is odd we need to prove that both y is odd and that (z+1) is odd i.e. z is even.

Statement 1: Tells us nothing about z. So insufficient.

Statement 2: Since z is even, (z+1) is odd. But tells us nothing about y. So insufficient.

Together: We can conclude that y and (z+1) are odd, so y(z+1) is odd.

Therefore answer is C.
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candace_washington1980
 Post subject: Re: If y and z are integers
 Post Posted: Thu Aug 06, 2009 12:29 pm 
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Wow you made that so simple, I was able to understand your explanation when I went back and looked at the question again. I really struggle with these DS questions. Thank you so much.
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furtadovinod
 Post subject: Re: If y and z are integers
 Post Posted: Thu Aug 06, 2009 9:08 pm 
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Posts: 28
candace_washington1980 wrote:
Wow you made that so simple, I was able to understand your explanation when I went back and looked at the question again. I really struggle with these DS questions. Thank you so much.


Can I ask what books/guides are you using for re-visiting all the basics? The manhattan books are great but you need to go through them slowly and understand every bit. No point rushing and also do all the Action Problems in there.

I am sure as time passes you will become better and better. So don't be disheartened.

There is one tip that can be useful: Every problem that you have solved try explaining the answer to yourself as you would to a class. That is talk loudly and explain every step. Its a great way for learning cause you will then start breaking up the problems into single sentences and that's usually much easier to solve problems rather than looking at the problem as a whole.

Did I make sense there? :)
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Ben Ku
 Post subject: Re: If y and z are integers
 Post Posted: Mon Aug 17, 2009 8:18 pm 
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ManhattanGMAT Staff


Posts: 823
Hi furtadovinod,

Thanks for the correct explanation. This problem as shown can be solved just by applying even/odd rules. I think also that your suggestion to "teach" problems is right on. In class, we try to highlight the importance of reviewing our work when we're done with a problem, give very detailed instructions how to do so. Teaching a problem out loud is certainly a helpful technique to master a topic. Thanks!

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Ben Ku
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ManhattanGMAT
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