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Source: — Data Sufficiency |
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by g_beatthegmat » Tue Jun 17, 2008 9:50 am
Should be (A).

From (A), we know that both x and y are prime numbers. Also, we already know that except number 2, all prime numbers are odd.

We have two cases now -

(i) Either both x and y are equal (not mentioned in the question so can not assume that they're not)
In this case, either both are 2, or non-2 prime numbers. In both the cases, x(y+1) = even.

(ii) Both x and y are not equal
In this case, if one is 2, then automatically x(y+1) = even. If both are non-2 prime numbers, then ODD*(ODD+1) = ODD*EVEN = EVEN.

Thus (a) is sufficient.


(b) is in sufficient because it doesn't tell anything about x.

Hope it helps.
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by Motherjane » Tue Jun 17, 2008 9:56 am
Hi atlantic,

First let us look at the problem statement.
x(y+1) is even or not?

We know that Even * Even = Even
or Even * odd = Even

So in this case we have the following conditions:

1) X is even and Y +1 is even
which means X is even and Y is odd
or
2) X is even and Y+1 is odd
which means X is even and Y is even
or
3) Y+1 is even and X is odd
which means Y is odd and X is odd

Coming to statement 1:
x & y are prime numbers.
All prime numbers are odd except 2.

So even if X is 2 and Y is any other odd number
or X&Y are both odd numbers, we know that X(Y+1) is even. Hence sufficient. AD

Statement 2:
y>7

Nothing abt x. Also we dont know whether y is even or odd.
Hence insufficient.

Answer (A).

Hope this is correct.
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by atlantic » Tue Jun 17, 2008 9:59 am
Thanks g_beatthegmat.

(A) was my choice also, but unfortunately not correct.

I'll post OA later. Anybody else?
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by vinod_ece66 » Tue Jun 17, 2008 10:24 am
Is the answer C

a) x and y prime numbers
the product can be both even or odd
for even x= 2 y=3
for odd y=2 x=3
so not sufficient
b) y>7
for even x=2 y=8
for odd x =3 y=9
not sufficient

however combining both we can say the y+1 term is always going to be

even.

hence even*even =even
or odd*even = even
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by g_beatthegmat » Tue Jun 17, 2008 10:31 am
Yes, I think vinod_ece66 is right. I overlooked the case when y could be = 2 and x non-2 prime.

C it is.
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by atlantic » Tue Jun 17, 2008 10:37 am
OA is C.

Now I understand my mistakes. There is only one way that x(y+1) can be odd with prime numbers, and that is when x=3 and y=2. All the others are obviously even. GMAT can be tricky.

Thanks guys.
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by Motherjane » Tue Jun 17, 2008 10:45 am
Thats correct!!

Haven't thought on these lines....

It is (C).

Thanks to all of ya.
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