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What is the value of x/yz? [#permalink]
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08 Mar 2009, 19:28
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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 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.
Thanks,
FinanceGuy25
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Last edited by Bunuel on 05 Jul 2013, 04:12, edited 1 time in total.
Edited the question and added the OA



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Re: Quant DS #90 [#permalink]
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08 Mar 2009, 20:09
financeguy25 wrote: 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.
Thanks,
FinanceGuy25 Note the red part: x/yz = x/[(2x*2x)/5] = 5x/4x^2 = 5/4x
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Re: Quant DS #90 [#permalink]
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08 Mar 2009, 20:28
we can write z = 2y/10
x = y/2
then (y/2) / ((y*y)/5) = 5/2y
so A is insufficient



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Re: Quant DS #90 [#permalink]
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08 Mar 2009, 21:01
Hey guys,
Thanks for the help...Thanks GMAT Tiger for pointing out the mistake, I forgot that 2x*2x=4x[Squared], not just 4x.
Thanks,
Financeguy25



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Re: What is the value of x / yz ? [#permalink]
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05 Jul 2013, 03:48
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 ? Please help !!



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Re: What is the value of x / yz ? [#permalink]
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05 Jul 2013, 04:40
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. 3. I don't know what question is Q15. Hope it helps.
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Re: What is the value of x / yz ? [#permalink]
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05 Jul 2013, 06:24
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.



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Re: What is the value of x / yz ? [#permalink]
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05 Jul 2013, 06:36
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.
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Collection of Questions: PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.
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Re: What is the value of x/yz? [#permalink]
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16 Sep 2016, 15:54
x/yz = ?
(1) x = y/2 and z = 2x/5
rewrite first eq as y = 2x and plug into original equation
x/[(2x)(2x/5)] = 5/4x > we still don't know the exact value of the original equation
(2) x/z = 5/2 and 1/y = 1/10
(5/2)(1/10) = 5/20 > we have a value!!!
Therefore, (2) is sufficient & B is the correct answer




Re: What is the value of x/yz?
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