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Hi guys, this was written before I saw the post on how to write mathematically correct in the forum.

DS #90:

My question about this problem has to do with part 1. The question is fairly simple: Why in part 1 can’t I make all signs match up and equal x? x/yz would then look like this in the way I did it: x / 2x*2x/5.

Here is what the book says:

(1) From this, z can be expressed in terms of y by substituting y/2 for x in the equation z=2x/5, which gives us z=2(2/5) / 5= y/5. The value of x/yz in terms of y is then y/2 / y(y/5) = y/2(5/y[squared])=5/2y. This expression cannot be evaluated further since no information is given about the value of y; not sufficient. (BCE).

Here is how I did it:

What I did is take x=y/2 and make it 2x=y (which Quant book did not do), which then gave me: x / 2x*2x/5, which gave me x / 4x/5, which equals 5x / 4x, and I get 5/4. Meaning I mark this as AD and move to part 2.

Part 2 is easier to solve because we just plug in: y=10, x=5, z=2. 5/10*2=1/4, for which I pick letter D as my answer because both solutions by them selves are sufficient.

One of the big problems I see with my answer is that in part 1 I got 5/4, and in part 2 I got ¼, which is a contradiction and according to MGMAT, I did something wrong.

Please explain this problem and let me know whether and if so, then why I am wrong.

Hi guys, this was written before I saw the post on how to write mathematically correct in the forum.

DS #90:

What is the value of x / yz ?

1. x=y/2 and z=2x/5 2. x/z=5/2 and 1/y=1/10

My question about this problem has to do with part 1. The question is fairly simple: Why in part 1 can’t I make all signs match up and equal x? x/yz would then look like this in the way I did it: x / 2x*2x/5.

Here is what the book says:

(1) From this, z can be expressed in terms of y by substituting y/2 for x in the equation z=2x/5, which gives us z=2(2/5) / 5= y/5. The value of x/yz in terms of y is then y/2 / y(y/5) = y/2(5/y[squared])=5/2y. This expression cannot be evaluated further since no information is given about the value of y; not sufficient. (BCE).

Here is how I did it:

What I did is take x=y/2 and make it 2x=y (which Quant book did not do), which then gave me: x / 2x*2x/5, which gave me x / 4x/5, which equals 5x / 4x, and I get 5/4. Meaning I mark this as AD and move to part 2.

Part 2 is easier to solve because we just plug in: y=10, x=5, z=2. 5/10*2=1/4, for which I pick letter D as my answer because both solutions by them selves are sufficient.

One of the big problems I see with my answer is that in part 1 I got 5/4, and in part 2 I got ¼, which is a contradiction and according to MGMAT, I did something wrong.

Please explain this problem and let me know whether and if so, then why I am wrong.

Re: What is the value of x / yz ? [#permalink]
05 Jul 2013, 02:48

financeguy25 wrote:

What is the value of x / yz ?

1. x=y/2 and z=2x/5 2. x/z=5/2 and 1/y=1/10

My question about this problem has to do with part 1. The question is fairly simple: Why in part 1 can’t I make all signs match up and equal x? x/yz would then look like this in the way I did it: x / 2x*2x/5.

Here is what the book says:

(1) From this, z can be expressed in terms of y by substituting y/2 for x in the equation z=2x/5, which gives us z=2(2/5) / 5= y/5. The value of x/yz in terms of y is then y/2 / y(y/5) = y/2(5/y[squared])=5/2y. This expression cannot be evaluated further since no information is given about the value of y; not sufficient. (BCE).

Hi Bunuel/Instructors, I have a doubt : (In red above) how can we cancel Y in numerator with Y in denominator, if the problem doesnt states: y IS NOT= 0. As the cancellation of Y(or for any variable) involves/multiplication or division by Y on both ends.

So far as I understand GMAT has marked statements in many DS Qs as insuffiecient on this ground stating "we cant divide by a variable unless it is mentioned as non zero"

I observed similar things in Q57(where m/n = 5/3 has been cross multiplied to 5n =3m) - they ignored the point that if m or n is 0 then the answer B wont be SUFFICIENT.

& Q15(in option B, they have ignored the case of x=0, which if true makes B not sufficient as it also the solution also involves division by a variable )

ARE there any particular scenarios in which we are allowed to divide/multiply varibles or transfer them by cross multiplication in an equation. OR where am i lacking in my basics ?

Re: What is the value of x / yz ? [#permalink]
05 Jul 2013, 03:40

Expert's post

p111 wrote:

financeguy25 wrote:

What is the value of x / yz ?

1. x=y/2 and z=2x/5 2. x/z=5/2 and 1/y=1/10

My question about this problem has to do with part 1. The question is fairly simple: Why in part 1 can’t I make all signs match up and equal x? x/yz would then look like this in the way I did it: x / 2x*2x/5.

Here is what the book says:

(1) From this, z can be expressed in terms of y by substituting y/2 for x in the equation z=2x/5, which gives us z=2(2/5) / 5= y/5. The value of x/yz in terms of y is then y/2 / y(y/5) = y/2(5/y[squared])=5/2y. This expression cannot be evaluated further since no information is given about the value of y; not sufficient. (BCE).

Hi Bunuel/Instructors, I have a doubt : (In red above) how can we cancel Y in numerator with Y in denominator, if the problem doesnt states: y IS NOT= 0. As the cancellation of Y(or for any variable) involves/multiplication or division by Y on both ends.

So far as I understand GMAT has marked statements in many DS Qs as insuffiecient on this ground stating "we cant divide by a variable unless it is mentioned as non zero"

I observed similar things in Q57(where m/n = 5/3 has been cross multiplied to 5n =3m) - they ignored the point that if m or n is 0 then the answer B wont be SUFFICIENT.

& Q15(in option B, they have ignored the case of x=0, which if true makes B not sufficient as it also the solution also involves division by a variable )

ARE there any particular scenarios in which we are allowed to divide/multiply varibles or transfer them by cross multiplication in an equation. OR where am i lacking in my basics ?

Please help !!

What is the value of x/yz?

(1) x = \frac{y}{2} and z = \frac{2x}{5}. If y=10, then x=5, z=2 and in this case \frac{x}{yz}=\frac{5}{20}=\frac{1}{4} BUT if y=20, then x=10, z=4 and in this case \frac{x}{yz}=\frac{10}{80}=\frac{1}{8}. Not sufficient.

(2) \frac{x}{z} = \frac{5}{2} and \frac{1}{y} = \frac{1}{10}. Multiply these two equations: \frac{x}{yz}=\frac{5}{20}. Sufficient.

Answer: B.

As for your questions: 1. Yes, it would be better if the stem stated that yz\neq{0}. If you don't state that, then x=y=z=0 satisfies the first statement and in this case x/(yz) is undefined.

2. As for m/n = 5/3. This equation already implies that neither of them is zero: if m=0, then m/n = 0 and not 5/3 and if n=0, then m/n is not defined, and not 5/3. So, in this case we can write 3m = 5n.

Re: What is the value of x / yz ? [#permalink]
05 Jul 2013, 05:24

Hi, Appologies for the PM, I wasnt aware.

Bunuel wrote:

As for your questions: 1. Yes, it would be better if the stem stated that yz\neq{0}. If you don't state that, then x=y=z=0 satisfies the first statement and in this case x/(yz) is undefined.

I think you understand my doubt. The scenarios(x,y,z=0) will make the solution undefined. Hence answer E. (Thats the way i solved, considering all scenarios) SO why are we not picking these values as well? The statement is meant to be true for all real numbers ~ RIGHT ? How to decide between option D(here) & option E for such questions in exam. Seeking lessons from your experience

Bunuel wrote:

3. I don't know what question is Q15.

It is question 15 from Data sufficiency & Q57 is question 57.

Re: What is the value of x / yz ? [#permalink]
05 Jul 2013, 05:36

Expert's post

p111 wrote:

Hi, Appologies for the PM, I wasnt aware.

Bunuel wrote:

As for your questions: 1. Yes, it would be better if the stem stated that yz\neq{0}. If you don't state that, then x=y=z=0 satisfies the first statement and in this case x/(yz) is undefined.

I think you understand my doubt. The scenarios(x,y,z=0) will make the solution undefined. Hence answer E. (Thats the way i solved, considering all scenarios) SO why are we not picking these values as well? The statement is meant to be true for all real numbers ~ RIGHT ? How to decide between option D(here) & option E for such questions in exam. Seeking lessons from your experience

Bunuel wrote:

3. I don't know what question is Q15.

It is question 15 from Data sufficiency & Q57 is question 57.

The answer to the question is neither D nor E, it's B.

(1) is not sufficient irrespective whether you have yz\neq{0} condition or not. The point is that, their solution for (1) is not 100% correct since it's not mentioned that yz\neq{0}.

(2) is still sufficient as shown in my post above.

As for Q15 and Q57: I don't know what questions are you talking about. Please post them.