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 Q51  V47
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Re: A 5 meter long wire is cut into two pieces. If the longer piece is the [#permalink]
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Hovkial wrote:
The original question is improperly framed and hence is not representative of a proper GMAT question. It is however helpful towards understanding the many nuances of this class of problems.


This is an official problem (from an early software edition of the GMATPrep tests) which tests the same concept as the question in this thread:

https://gmatclub.com/forum/a-point-is-a ... 51246.html

There isn't any ambiguity about how to interpret a question like this (on the GMAT, or in mathematics generally). A line (or 'wire') is continuous; it is not a finite collection of points, and if someone solves a problem like this by thinking of the line as a finite set of points, they'll only be approximating the answer.
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Re: A 5 meter long wire is cut into two pieces. If the longer piece is the [#permalink]
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The distance between any two points on a line has to be a non zero value and hence there can only be finite number of points on a line
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Re: A 5 meter long wire is cut into two pieces. If the longer piece is the [#permalink]
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SVaidyaraman wrote:
The distance between any two points on a line has to be a non zero value and hence there can only be finite number of points on a line


Your conclusion does not follow. The distance between any two distinct integers is nonzero, but they are infinite in number. Or if you want to take a bounded set, the distance between any two distinct fractions 1/n, where n is a positive integer, is nonzero, but they are also infinite in number.

A line is a mathematical construct. It is defined to be an infinite collection of points (unless you're working in niche areas of finite geometry that you'd only ever encounter in very advanced math classes, and which are irrelevant on the GMAT). You can find the definition of a line counterintuitive if you like, but that is the definition, both on the GMAT and in standard mathematics. I don't understand how you think you might be helping test takers reading this thread by insisting a line is something other than what the GMAT defines it to be.
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Re: A 5 meter long wire is cut into two pieces. If the longer piece is the [#permalink]
IanStewart wrote:

This is an official problem (from an early software edition of the GMATPrep tests) which tests the same concept as the question in this thread:

https://gmatclub.com/forum/a-point-is-a ... 51246.html

There isn't any ambiguity about how to interpret a question like this (on the GMAT, or in mathematics generally). A line (or 'wire') is continuous; it is not a finite collection of points, and if someone solves a problem like this by thinking of the line as a finite set of points, they'll only be approximating the answer.



This is not an official GMAT problem. This is a problem from the GMATCLUB tests collection. The linkage you have provided is to some different problem that has a different formulation. Both appear to be from the GMATCLUB tests collection.

A question is ambiguous if multiple and many testtakers reasonably interpret the problem in myriad ways to arrive at varying answers. It is erroneous to state that this is how testtakers "should" be interpreting something. We look at real world usage data and accordingly adjust question writing processes. Good test design standards examine real-world usages and do not rely on "shoulda coulda" rules. It is the responsibility of the test writer to unambiguously write the question.

The official GMAT follows good test design procedures and that is an important reason why their questions are the gold standard. This does not mean they have no slip-ups, but those are very few in number compared to those from third-parties.
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Re: A 5 meter long wire is cut into two pieces. If the longer piece is the [#permalink]
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Hovkial wrote:
This is not an official GMAT problem. This is a problem from the GMATCLUB tests collection. The linkage you have provided is to some different problem that has a different formulation. Both appear to be from the GMATCLUB tests collection.


The question in this thread is from the gmatclub tests. The question I linked to certainly is not; it's from the 2007 edition of the GMATPrep software test. You can believe that or not, as you like.

I agree with you that there are very often issues with wording in prep company questions. I frequently point out those issues on this forum. But I'm not able to understand what ambiguity you detect in this particular problem, nor do I agree with your characterization of what makes a question ambiguous. A GMAT question would be considered unambiguous if mathematicians agreed on its meaning. I'm not sure if you find ambiguous the wording in the question I linked to (which is essentially the same as the one in this thread, but simpler), but the wording in that link is the wording they use on the real test, so it would not be considered ambiguous on the GMAT.
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Re: A 5 meter long wire is cut into two pieces. If the longer piece is the [#permalink]
IanStewart wrote:

The question in this thread is from the gmatclub tests. The question I linked to certainly is not; it's from the 2007 edition of the GMATPrep software test. You can believe that or not, as you like.

I agree with you that there are very often issues with wording in prep company questions. I frequently point out those issues on this forum. But I'm not able to understand what ambiguity you detect in this particular problem, nor do I agree with your characterization of what makes a question ambiguous. A GMAT question would be considered unambiguous if mathematicians agreed on its meaning. I'm not sure if you find ambiguous the wording in the question I linked to (which is essentially the same as the one in this thread, but simpler), but the wording in that link is the wording they use on the real test, so it would not be considered ambiguous on the GMAT.


The linked question has an unspecified source. It could be an official question or not, though I have remember seeing similar types of official questions.

The discussion comments pertain to the GMATCLUB question here. Nothing here relates to that linked question or its wording, which is irrelevant to me.

"A GMAT question would be considered unambiguous if mathematicians agreed on its meaning."

The GMAT test is not taken solely by mathematicians. It is taken by members of the general public. Their interpretations while answering the question are important determinants of whether a question is framed properly (e.g., unambiguously). What some group of mathematicians think is really not relevant to the practical use of this test.
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Re: A 5 meter long wire is cut into two pieces. If the longer piece is the [#permalink]
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IanStewart wrote:
SVaidyaraman wrote:
The distance between any two points on a line has to be a non zero value and hence there can only be finite number of points on a line


Your conclusion does not follow. The distance between any two distinct integers is nonzero, but they are infinite in number. Or if you want to take a bounded set, the distance between any two distinct fractions 1/n, where n is a positive integer, is nonzero, but they are also infinite in number.

A line is a mathematical construct. It is defined to be an infinite collection of points (unless you're working in niche areas of finite geometry that you'd only ever encounter in very advanced math classes, and which are irrelevant on the GMAT). You can find the definition of a line counterintuitive if you like, but that is the definition, both on the GMAT and in standard mathematics. I don't understand how you think you might be helping test takers reading this thread by insisting a line is something other than what the GMAT defines it to be.

I go by pure logic. I want to be bothered less by specific knowledge if logic can help me arrive at the answer. I am not hindering the test takers in any way . I am trying to be true to the spirit of GMAT which is supposed to predominantly test logic.
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Re: A 5 meter long wire is cut into two pieces. If the longer piece is the [#permalink]
EMPOWERgmatRichC wrote:
Hi All,

This question is vaguely worded in spots, but I've offered an explanation that matches the 'intent' of the question. There's an aspect to this question that many Test Takers would miss: regardless of where on the wire the 'cut' is made, the 'longer' piece is the one that's used to make the square. Thus, unless you cut the wire exactly in the middle, there are will always be two versions of each measure.

For example...
if you cut the wire at the 1m "mark", you'll have a 1m piece and a 4m piece
if you cut the wire at the 4m "mark", you'll also have a 1m piece and a 4m piece

Since area of a square is (side)^2, for the area to be GREATER than 1 m^2, the side lengths have to be GREATER than 1. By extension, this means that the perimeter would have to be GREATER than 4. Thus, any cut that produces a longer piece that is greater than 4m will satisfy what this question is asking for...

Cutting LESS than 1m will do it.
Cutting MORE than 4m will also do it.

That's approximately 2m of a 5m wire... 2/5

Final Answer:

GMAT assassins aren't born, they're made,
Rich


OK went through a good chunk of the posts in this thread. Clearly lots of deep discussion. For simplicity, the idea here is we get 40% because the rope can be cut in 2 places.

A vast oversimplification?
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Re: A 5 meter long wire is cut into two pieces. If the longer piece is the [#permalink]
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CEdward wrote:

OK went through a good chunk of the posts in this thread. Clearly lots of deep discussion. For simplicity, the idea here is we get 40% because the rope can be cut in 2 places.

A vast oversimplification?


I don't quite follow your reasoning, so it may or may not be right. The rope can be cut in infinitely many places, not just in two. The answer is 2/5 because there if we make the cut in a 2 meter stretch of rope, we can create the square the question asks about, and we're choosing those 2 meters from a 5 meter stretch of rope in total.

The post you quote isn't correct in two places -- the question is not vaguely worded, and the answer to the question is exact; it is not "approximately... 2/5". It's probably better to refer to the solutions from Bunuel, Karishma, or me earlier in the thread if you need further explanation.

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Re: A 5 meter long wire is cut into two pieces. If the longer piece is the [#permalink]
Hi,

Can someone help with why my approach is incorrect?

I assumed one side to be x and the other to be 5-x. (x is assumed to be the longer side)

The side of the square is therefore x/4 and area is \( x2\)/16.

For area to be >1, \( x2\)/16 should be greater than 1 or |x|>4.

Since 5-x>0, x<5. Therefore, favorable values of x are between 4 and 5. The whole rope is 5 units. Therefore ans = 1/5
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Re: A 5 meter long wire is cut into two pieces. If the longer piece is the [#permalink]
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nikhitam wrote:
I assumed one side to be x and the other to be 5-x. (x is assumed to be the longer side)


When you assume x is the longer piece of rope, x must be between 2.5 and 5 meters. So your denominator should be 2.5, and not 5. Otherwise your solution is good.

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Re: A 5 meter long wire is cut into two pieces. If the longer piece is the [#permalink]
IanStewart wrote:
nikhitam wrote:
I assumed one side to be x and the other to be 5-x. (x is assumed to be the longer side)


When you assume x is the longer piece of rope, x must be between 2.5 and 5 meters. So your denominator should be 2.5, and not 5. Otherwise your solution is good.

Posted from my mobile device


Understood! Thanks a lot;
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Re: A 5 meter long wire is cut into two pieces. If the longer piece is the [#permalink]
MisterEko wrote:
A 5 meter long wire is cut into two pieces. If the longer piece is then used to form a perimeter of a square, what is the probability that the area of the square will be more than 1 if the original wire was cut at an arbitrary point?

A) 1/6
B) 1/5
C) 3/10
D) 1/3
E) 2/5



Hello Bunuel

I having trouble with this question by approaching it in a different manner. Can you please point out my mistake? Thanks.

As the square is formed with the help of the larger piece, we can say that \(P\in(2.5,5)\)
If the side of the square is assumed to be \(a\), then \(4a\in(2.5,5)\)
hence,
\(a\in(\frac{5}{8},\frac{5}{4})\)

Area of square, \(a^2\in(\frac{25}{64},\frac{25}{16})\)

hence, probability of \(a^2\) greater than \(1\) will be \(\frac{\frac{25}{16}-1}{\frac{25}{16}-\frac{25}{64}}\)

hence, \(\frac{36}{75}\)

It is nowhere close to the given answer. Can you please point out the mistake?
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Re: A 5 meter long wire is cut into two pieces. If the longer piece is the [#permalink]
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amoljain wrote:
Area of square, \(a^2\in(\frac{25}{64},\frac{25}{16})\)

hence, probability of \(a^2\) greater than \(1\) will be \(\frac{\frac{25}{16}-1}{\frac{25}{16}-\frac{25}{64}}\)

Can you please point out the mistake?


It's when you squared things that the solution becomes incorrect, though it's quite a subtle error. We're choosing a random length, not, as your solution assumes, a random area, and lengths and areas are not distributed the same way.

You can see why this is a problem with a much simpler example: say you choose a random real number from between 0 and 10, and you use that number as the length of the side of a square. Say you want to know the probability the area of this square will be less than 25. That will happen if we pick a number less than 5, so it will happen 5/10 = 1/2 of the time. But if you square things first, and conclude "the area is something random between 0 and 100", you'd think the answer was 25/100 = 1/4. The problem is that the distance between squares grows the bigger the numbers you're squaring. The big squares, in this example, are rarer than the small ones (more dramatically - an area in my simpler example between 0 and 1 isn't all that unlikely, we'll get that 1/10 of the time, but an area between 99 and 100 is very unlikely - the probability is close to 1/200).

If you complete your solution without squaring anything, then it will become correct (if you just stop once you find lengths, and ask the probability the piece of rope is long enough, your solution will be right).
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Re: A 5 meter long wire is cut into two pieces. If the longer piece is the [#permalink]
IanStewart wrote:
amoljain wrote:
Area of square, \(a^2\in(\frac{25}{64},\frac{25}{16})\)

hence, probability of \(a^2\) greater than \(1\) will be \(\frac{\frac{25}{16}-1}{\frac{25}{16}-\frac{25}{64}}\)

Can you please point out the mistake?


It's when you squared things that the solution becomes incorrect, though it's quite a subtle error. We're choosing a random length, not, as your solution assumes, a random area, and lengths and areas are not distributed the same way.

You can see why this is a problem with a much simpler example: say you choose a random real number from between 0 and 10, and you use that number as the length of the side of a square. Say you want to know the probability the area of this square will be less than 25. That will happen if we pick a number less than 5, so it will happen 5/10 = 1/2 of the time. But if you square things first, and conclude "the area is something random between 0 and 100", you'd think the answer was 25/100 = 1/4. The problem is that the distance between squares grows the bigger the numbers you're squaring. The big squares, in this example, are rarer than the small ones (more dramatically - an area in my simpler example between 0 and 1 isn't all that unlikely, we'll get that 1/10 of the time, but an area between 99 and 100 is very unlikely - the probability is close to 1/200).

If you complete your solution without squaring anything, then it will become correct (if you just stop once you find lengths, and ask the probability the piece of rope is long enough, your solution will be right).


Thanks IanStewart, this helps a lot.
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Re: A 5 meter long wire is cut into two pieces. If the longer piece is the [#permalink]
Here is my solution:

The length of the wire is 5m. We are given that one wire is longer than the other wire, Lw>2.5 whil Sw <2.5.

The length of the long wire can theoretically range from 2.5<Lw<5.

The Lw is made into a perimeter of the square (s), therefore we can write Lw as 4s.

We can write 2.5<Lw<5 as:

2.5<4s<5

Dividing both the sides of the inequality by 4 gives us:

0.625<s<1.25

For the Area of the square to be greater than 1, we have to consider s only for 1<s<1.25 while the total range of possibilities of s are from 1.25-0.625=0.625

Prob = 0.25/0.625

This can be rewritten as 250/625 - Simplify this by dividing it by 25

10/25

2/5

Option E is the answer
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Re: A 5 meter long wire is cut into two pieces. If the longer piece is the [#permalink]
avigutman please add your way of solving the above question. I have tried to solve it by ratios but I am little bit confused.
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