Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized for You

we will pick new questions that match your level based on your Timer History

Track Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

It appears that you are browsing the GMAT Club forum unregistered!

Signing up is free, quick, and confidential.
Join other 500,000 members and get the full benefits of GMAT Club

Registration gives you:

Tests

Take 11 tests and quizzes from GMAT Club and leading GMAT prep companies such as Manhattan GMAT,
Knewton, and others. All are free for GMAT Club members.

Applicant Stats

View detailed applicant stats such as GPA, GMAT score, work experience, location, application
status, and more

Books/Downloads

Download thousands of study notes,
question collections, GMAT Club’s
Grammar and Math books.
All are free!

Thank you for using the timer!
We noticed you are actually not timing your practice. Click the START button first next time you use the timer.
There are many benefits to timing your practice, including:

the answer is E (this is the answer that you get if you solve the equation), but I was just wondering why the answer couldn't also be (A) because if you plug the variables into the equation you find that the left side equals the right side.

Re: Algebra question on the GMAC practice test [#permalink]

Show Tags

16 Sep 2009, 12:55

2

This post received KUDOS

dhushan wrote:

If xy+z = x(y+z), which of the following must be true?

1. x=0 and z=0 2. x=1 and y=1 3. y=1 and z=0 4. x=1 or y=0 5. x=1 or z=0

the answer is E (this is the answer that you get if you solve the equation), but I was just wondering why the answer couldn't also be (A) because if you plug the variables into the equation you find that the left side equals the right side.

Re: Algebra question on the GMAC practice test [#permalink]

Show Tags

16 Sep 2009, 13:15

1

This post received KUDOS

dhushan wrote:

If xy+z = x(y+z), which of the following must be true?

1. x=0 and z=0 2. x=1 and y=1 3. y=1 and z=0 4. x=1 or y=0 5. x=1 or z=0

the answer is E (this is the answer that you get if you solve the equation), but I was just wondering why the answer couldn't also be (A) because if you plug the variables into the equation you find that the left side equals the right side.

i.e. xy + z = x(y+z) (0)y + 0 = 0(y+0) 0 = 0

what's wrong with my logic here.

I understand this, but what's wrong with plugging in 0, you still get the right hand equalling the left hand side, i.e. 0 = 0.

Re: Algebra question on the GMAC practice test [#permalink]

Show Tags

17 Sep 2009, 03:29

2

This post received KUDOS

dhushan wrote:

dhushan wrote:

If xy+z = x(y+z), which of the following must be true?

1. x=0 and z=0 2. x=1 and y=1 3. y=1 and z=0 4. x=1 or y=0 5. x=1 or z=0

the answer is E (this is the answer that you get if you solve the equation), but I was just wondering why the answer couldn't also be (A) because if you plug the variables into the equation you find that the left side equals the right side.

i.e. xy + z = x(y+z) (0)y + 0 = 0(y+0) 0 = 0

what's wrong with my logic here.

I understand this, but what's wrong with plugging in 0, you still get the right hand equalling the left hand side, i.e. 0 = 0.

I did not find any reason except this: xy+z=x(y+z) can be rewritten as (xy+z)/x=(y+z) if x = 0 then the LHS will be infinite.

Re: Algebra question on the GMAC practice test [#permalink]

Show Tags

17 Sep 2009, 08:55

1

This post received KUDOS

maliyeci wrote:

dhushan wrote:

dhushan wrote:

If xy+z = x(y+z), which of the following must be true?

1. x=0 and z=0 2. x=1 and y=1 3. y=1 and z=0 4. x=1 or y=0 5. x=1 or z=0

the answer is E (this is the answer that you get if you solve the equation), but I was just wondering why the answer couldn't also be (A) because if you plug the variables into the equation you find that the left side equals the right side.

i.e. xy + z = x(y+z) (0)y + 0 = 0(y+0) 0 = 0

what's wrong with my logic here.

I understand this, but what's wrong with plugging in 0, you still get the right hand equalling the left hand side, i.e. 0 = 0.

I did not find any reason except this: xy+z=x(y+z) can be rewritten as (xy+z)/x=(y+z) if x = 0 then the LHS will be infinite.

That makes sense. I guess plugging in answers is not always the best options in these situations. Thanks.

Re: Algebra question on the GMAC practice test [#permalink]

Show Tags

18 Sep 2009, 11:51

2

This post received KUDOS

this equation can be solved directly xy + z = x(y+z) xy + z = xy +xz z=xz or xz - z = 0 z(x-1) = 0 z= 0 or x = 1.

its all ways better to solve if the equations are simple like mentioned above....substituting values will be better if they r more complex invloving 3 or 4 variables .

If xy+z = x(y+z), which of the following must be true?

1. x=0 and z=0 2. x=1 and y=1 3. y=1 and z=0 4. x=1 or y=0 5. x=1 or z=0

the answer is E (this is the answer that you get if you solve the equation), but I was just wondering why the answer couldn't also be (A) because if you plug the variables into the equation you find that the left side equals the right side.

i.e. xy + z = x(y+z) (0)y + 0 = 0(y+0) 0 = 0

what's wrong with my logic here.

I understand this, but what's wrong with plugging in 0, you still get the right hand equalling the left hand side, i.e. 0 = 0.

The question asks what *must* be true. Sure, it *could* be true that x = 0 and z = 0, but that does not *need* to be true. We might instead have that x = 1 and z = 5, for example. If you plug in the numbers from the answer choices here, all you learn is what *may* be true, not what *must* be true.
_________________

GMAT Tutor in Toronto

If you are looking for online GMAT math tutoring, or if you are interested in buying my advanced Quant books and problem sets, please contact me at ianstewartgmat at gmail.com

Re: Algebra question on the GMAC practice test [#permalink]

Show Tags

17 Sep 2013, 01:33

dhushan wrote:

If xy+z = x(y+z), which of the following must be true?

1. x=0 and z=0 2. x=1 and y=1 3. y=1 and z=0 4. x=1 or y=0 5. x=1 or z=0

the answer is E (this is the answer that you get if you solve the equation), but I was just wondering why the answer couldn't also be (A) because if you plug the variables into the equation you find that the left side equals the right side.

i.e. xy + z = x(y+z) (0)y + 0 = 0(y+0) 0 = 0

what's wrong with my logic here.

The reason why the above logic is wrong is after simplifying the expression, we have, xz=z. For this to be true either x has to be 1 or z has to be 0 but choice A says that both x=0 and z=0 need to be true. We know that need not be the case.

For "which must be true" questions, if more than 1 choice seem to be correct, pick the one with the least restrictions.
_________________

If xy+z = x(y+z), which of the following must be true?

1. x=0 and z=0 2. x=1 and y=1 3. y=1 and z=0 4. x=1 or y=0 5. x=1 or z=0

\(xy+z=x(y+z)\) --> \(xy+z=xy+xz\) --> \(xy\) cancels out --> \(xz-z=0\) --> \(z(x-1)=0\) --> either \(z=0\) (in this case \(x\) can take ANY value) OR \(x=1\) (in this case \(z\) can take ANY value).

Answer: E.

To elaborate more: As the expression with \(y\) cancels out, we can say that given expression \(xy+z=x(y+z)\) does not depend on value of \(y\). Which means that \(y\) can take ANY value. So all answer choices which specify the exact value of \(y\) are wrong.

Next: "AND" in answer choices means that BOTH values must be true, but "OR" in answer choices means that EITHER value must be true.

There’s something in Pacific North West that you cannot find anywhere else. The atmosphere and scenic nature are next to none, with mountains on one side and ocean on...

This month I got selected by Stanford GSB to be included in “Best & Brightest, Class of 2017” by Poets & Quants. Besides feeling honored for being part of...

Joe Navarro is an ex FBI agent who was a founding member of the FBI’s Behavioural Analysis Program. He was a body language expert who he used his ability to successfully...