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M21-30 Mistake?

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M21-30 Mistake? [#permalink] New post 19 Feb 2013, 03:01
Is x^2*y^3*z>0?

(1) yz>0

(2) xz<0

The official answer from Gmat Club is C.

I strongly think the correct answer is A and here is why:

From statement 1 y & z have the same sign wither both - or both +. This mean that the part of the equation given y^3*z is positive. And since x^ has to be positive we can conclude that statement 1 is sufficient.

For me answer is A

What do you think?

Cheers
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Re: M21-30 Mistake? [#permalink] New post 19 Feb 2013, 03:28
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Expert's post
wieseljonas wrote:
Is x^2*y^3*z>0?

(1) yz>0

(2) xz<0

The official answer from Gmat Club is C.

I strongly think the correct answer is A and here is why:

From statement 1 y & z have the same sign wither both - or both +. This mean that the part of the equation given y^3*z is positive. And since x^ has to be positive we can conclude that statement 1 is sufficient.

For me answer is A

What do you think?

Cheers


Is x^2*y^3*z>0?

Inequality x^2*y^3*z>0 to be true:
1. y and z must be either both positive or both negative, so they must have the same sign (in this case y^3*z will be positive);
AND
2. x must not be zero (in this case x^2 will be positive).

(1) yz>0. From this statement it follows that y and z are either both positive or both negative, so the first condition is satisfied. But we don't know about x (the second condition). Not sufficient.

(2) xz<0. From this statement it follows that x\neq{0}, so the second condition is satisfied. Don't know about the signs of y and z (the first condition). Not sufficient.

(1)+(2) Both conditions are satisfied. Sufficient.

Answer: C.

Similar questions to practice:
is-x-2-y-5-z-0-1-xz-y-0-2-y-z-98341.html
is-x-7-y-2-z-3-0-1-yz-0-2-xz-127692.html
m21-q30-96613.html

Hope it helps.
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Joined: 05 Nov 2012
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Re: M21-30 Mistake? [#permalink] New post 19 Feb 2013, 03:30
Ok thanks.

It's the x must not be zero that I missed! Thanks you!
Re: M21-30 Mistake?   [#permalink] 19 Feb 2013, 03:30
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