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# If P is a set of integers and 3 is in P, is every positive

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Re: If P is a set of integers and 3 is in P, is every positive [#permalink]
Bunuel plz help. I m stuck here, how does st (1) ensure that just +ve multiples of 3 are in set P? For instance if it has -6, than 3 + -6 =-3, is also in that set, so the statement holds true but it has -ve multiples within the set. So I answered E due to the condition of "+ve multiples"
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Re: If P is a set of integers and 3 is in P, is every positive [#permalink]
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Bunuel plz help. I m stuck here, how does st (1) ensure that just +ve multiples of 3 are in set P? For instance if it has -6, than 3 + -6 =-3, is also in that set, so the statement holds true but it has -ve multiples within the set. So I answered E due to the condition of "+ve multiples"

Please pay attention to the part in red:
If P is a set of integers and 3 is in P, is every positive multiple of 3 in P?

Positive multiples of 3 are: 3, 6, 9, 12, 15, ... The question asks whether ALL these numbers are in the set P, taking into account that 3 is in this set.

(1) For any integer in P, the sum of 3 and that integer is also in P --> if $$x$$ is in the set, so is $$x+3$$ --> we know 3 is in P, hence $$3+3=6$$ is also in, and as 6 is in so is $$6+3=9$$, and so on. Which means that ALL positive multiples of 3 are in the set P. Sufficient.

Side note: above does not mean that only positive multiples of 3 are in P, there can be other numbers but we are only interested in them.

(2) For any integer in P, that integer minus 3 is also in P --> if $$x$$ is in the set, so is $$x-3$$ --> we know 3 is in P, hence $$3-3=0$$ is also in and as 0 is in, so is $$0-3=-3$$, and so on. So we are not sure whether all positive multiples of 3 are in P, all we know that there will be following numbers: 3, 0, -3, -6, -9, -12, ... Not sufficient.

The question does NOT ask whether P consists ONLY of positive multiples of 3. It asks whether every positive multiple of 3 in P.
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Re: If P is a set of integers and 3 is in P, is every positive [#permalink]
Thanks. The last line made it crystal clear
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Re: If P is a set of integers and 3 is in P, is every positive [#permalink]
Bunuel wrote:
Caffmeister wrote:
If P is a set of integers and 3 is in P, is every positive multiple of 3 in P?

(1) For any integer in P, the sum of 3 and that integer is also in P.

(2) For any integer in P, that integer minus 3 is also in P.

I had difficulty with this question because of the wording, I wasn't sure what they were looking for exactly, and I didn't find the explanation in the book to be sufficient. If anyone can break it down into an easier explanation I'd apprecaite it.

If P is a set of integers and 3 is in P, is every positive multiple of 3 in P?

Positive multiples of 3 are: 3, 6, 9, 12, 15, ... The question asks whether ALL these numbers are in the set P, taking into account that 3 is in this set.

(1) For any integer in P, the sum of 3 and that integer is also in P --> if $$x$$ is in the set, so is $$x+3$$ --> we know 3 is in P, hence $$3+3=6$$ is also in, and as 6 is in so is $$6+3=9$$, and so on. Which means that ALL positive multiples of 3 are in the set P. Sufficient.

Side note: above does not mean that only positive multiples of 3 are in P, there can be other numbers but we are only interested in them.

(2) For any integer in P, that integer minus 3 is also in P --> if $$x$$ is in the set, so is $$x-3$$ --> we know 3 is in P, hence $$3-3=0$$ is also in and as 0 is in, so is $$0-3=-3$$, and so on. So we are not sure whether all positive multiples of 3 are in P, all we know that there will be following numbers: 3, 0, -3, -6, -9, -12, ... Not sufficient.

Hope it's clear.

I'm not very clear with this answer. If your statement 2 can state like that, how didn't you question statement 1 in the same way? It means that we're not sure about whether set P contains min number like 0, 3, 6. Set P can start from 500 for example. In that case not every multiple of 3 is in the set P. Insufficient
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Re: If P is a set of integers and 3 is in P, is every positive [#permalink]
hoangphuc wrote:
Bunuel wrote:
Caffmeister wrote:
If P is a set of integers and 3 is in P, is every positive multiple of 3 in P?

(1) For any integer in P, the sum of 3 and that integer is also in P.

(2) For any integer in P, that integer minus 3 is also in P.

I had difficulty with this question because of the wording, I wasn't sure what they were looking for exactly, and I didn't find the explanation in the book to be sufficient. If anyone can break it down into an easier explanation I'd apprecaite it.

If P is a set of integers and 3 is in P, is every positive multiple of 3 in P?

Positive multiples of 3 are: 3, 6, 9, 12, 15, ... The question asks whether ALL these numbers are in the set P, taking into account that 3 is in this set.

(1) For any integer in P, the sum of 3 and that integer is also in P --> if $$x$$ is in the set, so is $$x+3$$ --> we know 3 is in P, hence $$3+3=6$$ is also in, and as 6 is in so is $$6+3=9$$, and so on. Which means that ALL positive multiples of 3 are in the set P. Sufficient.

Side note: above does not mean that only positive multiples of 3 are in P, there can be other numbers but we are only interested in them.

(2) For any integer in P, that integer minus 3 is also in P --> if $$x$$ is in the set, so is $$x-3$$ --> we know 3 is in P, hence $$3-3=0$$ is also in and as 0 is in, so is $$0-3=-3$$, and so on. So we are not sure whether all positive multiples of 3 are in P, all we know that there will be following numbers: 3, 0, -3, -6, -9, -12, ... Not sufficient.

Hope it's clear.

I'm not very clear with this answer. If your statement 2 can state like that, how didn't you question statement 1 in the same way? It means that we're not sure about whether set P contains min number like 0, 3, 6. Set P can start from 500 for example. In that case not every multiple of 3 is in the set P. Insufficient

We know that 3 is in the set. From (1) we also know that if any integer is in the set, then (that integer) + 3 is also in the set. Since 3 is in the set, then so must be 3 + 3 = 6. If 6 is in the set, then so must be 6 + 3 = 9, and so on.
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Re: If P is a set of integers and 3 is in P, is every positive [#permalink]
Hi Bunnel,

I understand how A is right, but I think B should also works fine. Because the second statement clearly mentions for every
Integer minus three results in that set. So now for. the given question 3 is already present, and that we need 6 in the set to get the result 3. So we only need multiples of 3 in positives too.
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Re: If P is a set of integers and 3 is in P, is every positive [#permalink]
gvrk_77 wrote:
Hi Bunnel,

I understand how A is right, but I think B should also works fine. Because the second statement clearly mentions for every
Integer minus three results in that set. So now for. the given question 3 is already present, and that we need 6 in the set to get the result 3. So we only need multiples of 3 in positives too.

Not so. You cannot say that if 3 is there than 6 must also be there. What if the set is {3, 0, -3, -6, -9, -12, ... } So, basically what if 3 is the source integer?

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Hope this helps.
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Re: If P is a set of integers and 3 is in P, is every positive [#permalink]
Bunuel wrote:
gvrk_77 wrote:
Hi Bunnel,

I understand how A is right, but I think B should also works fine. Because the second statement clearly mentions for every
Integer minus three results in that set. So now for. the given question 3 is already present, and that we need 6 in the set to get the result 3. So we only need multiples of 3 in positives too.

Not so. You cannot say that if 3 is there than 6 must also be there. What if the set is {3, 0, -3, -6, -9, -12, ... } So, basically what if 3 is the source integer?

Similar questions to practice:
https://gmatclub.com/forum/for-a-certai ... 36580.html
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https://gmatclub.com/forum/k-is-a-set-o ... 03005.html
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https://gmatclub.com/forum/for-a-certain ... 36580.html
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Hope this helps.

Hi bunnel,
So now as you’ve mentioned that what if the set consists of {3, 0, -3, -6, -9, -12, ... } is possible solution, then there is only one positive number which is multiple of 3 and that itself will satisfy the condition. We can’t write the set as {3, 2, 1, 0, -3, -6, -9, -12, ... }. Please help what I’m missing

Thanks,
Rajesh
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Re: If P is a set of integers and 3 is in P, is every positive [#permalink]
gvrk_77 wrote:
Bunuel wrote:
gvrk_77 wrote:
Hi Bunnel,

I understand how A is right, but I think B should also works fine. Because the second statement clearly mentions for every
Integer minus three results in that set. So now for. the given question 3 is already present, and that we need 6 in the set to get the result 3. So we only need multiples of 3 in positives too.

Not so. You cannot say that if 3 is there than 6 must also be there. What if the set is {3, 0, -3, -6, -9, -12, ... } So, basically what if 3 is the source integer?

Similar questions to practice:
https://gmatclub.com/forum/for-a-certai ... 36580.html
https://gmatclub.com/forum/a-set-of-num ... 98829.html
https://gmatclub.com/forum/k-is-a-set-o ... 03005.html
https://gmatclub.com/forum/k-is-a-set-o ... 96907.html
https://gmatclub.com/forum/for-a-certain ... 36580.html
https://gmatclub.com/forum/for-a-certai ... 61920.html

Hope this helps.

Hi bunnel,
So now as you’ve mentioned that what if the set consists of {3, 0, -3, -6, -9, -12, ... } is possible solution, then there is only one positive number which is multiple of 3 and that itself will satisfy the condition. We can’t write the set as {3,2, 1, 0, -3, -6, -9, -12, ... }. Please help what I’m missing

Thanks,
Rajesh

The question asks: is every positive multiple of 3 in P? So, basically the question asks whether 3, 6, 9, 12, 15, ... ALL positive multiple of 3 are is set P. It's certainly possible ALL of them to be in the set, if the set is {..., -12, -9, -6, -3, 0, 3, 6, 9, 12, ...} nothing prevents this set to be an actual one (answer YES) but it's also possible the set to be {3, 0, -3, -6, -9, -12, ... } (answer NO).
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Re: If P is a set of integers and 3 is in P, is every positive [#permalink]
I would go for E as well...

Imagine that the set mentioned is the set of all integers:
[...,-3,-2,-1,0,1,2,3,...]
Or it is a set of integers n, where n = 0+3k or n = 1+3k, k being any intenger:
[...-4,-3,-1,0,2,3,5,...]

In this case, the set satisfy statement a and b, but not every number in the set is a multiple of 3
so statement a and b are insufficient
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Re: If P is a set of integers and 3 is in P, is every positive [#permalink]
I would go for E as well...

Imagine that the set mentioned is the set of all integers:
[...,-3,-2,-1,0,1,2,3,...]
Or it is a set of integers n, where n = 0+3k or n = 1+3k, k being any intenger:
[...-4,-3,-1,0,2,3,5,...]

In this case, the set satisfy statement a and b, but not every number in the set is a multiple of 3
so statement a and b are insufficient

The question does not ask whether all the numbers in the set are multiple of 3. The question asks whether all positive multiples of 3 are in the set. The correct answer to the question is A, not E. Please read the discussion above carefully and follow the links provided.

Hope it helps.
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Re: If P is a set of integers and 3 is in P, is every positive [#permalink]
Bunuel wrote:
I would go for E as well...

Imagine that the set mentioned is the set of all integers:
[...,-3,-2,-1,0,1,2,3,...]
Or it is a set of integers n, where n = 0+3k or n = 1+3k, k being any intenger:
[...-4,-3,-1,0,2,3,5,...]

In this case, the set satisfy statement a and b, but not every number in the set is a multiple of 3
so statement a and b are insufficient

The question does not ask whether all the numbers in the set are multiple of 3. The question asks whether all positive multiples of 3 are in the set. The correct answer to the question is A, not E. Please read the discussion above carefully and follow the links provided.

Hope it helps.

Bunuel

I am not clear how 2nd statement is insufficient. According to me second statement should be sufficient as it has all the elements that are Element-3,

If there is an infinitely large number that is a multiple of 3 say 3333333333 then it will have 3333333330 in the set but it will not have 3333333336 in the set. If it has 3333333336 in the set it will not have 3333333339 in the set.

It does not guarantee that all positive multiple of 3 are there in the set as all positive multiple can tend to infinity.

Hence the statement 2 should be sufficient.

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Re: If P is a set of integers and 3 is in P, is every positive [#permalink]
Mudit27021988 wrote:
Bunuel wrote:
I would go for E as well...

Imagine that the set mentioned is the set of all integers:
[...,-3,-2,-1,0,1,2,3,...]
Or it is a set of integers n, where n = 0+3k or n = 1+3k, k being any intenger:
[...-4,-3,-1,0,2,3,5,...]

In this case, the set satisfy statement a and b, but not every number in the set is a multiple of 3
so statement a and b are insufficient

The question does not ask whether all the numbers in the set are multiple of 3. The question asks whether all positive multiples of 3 are in the set. The correct answer to the question is A, not E. Please read the discussion above carefully and follow the links provided.

Hope it helps.

Bunuel

I am not clear how 2nd statement is insufficient. According to me second statement should be sufficient as it has all the elements that are Element-3,

If there is an infinitely large number that is a multiple of 3 say 3333333333 then it will have 3333333330 in the set but it will not have 3333333336 in the set. If it has 3333333336 in the set it will not have 3333333339 in the set.

It does not guarantee that all positive multiple of 3 are there in the set as all positive multiple can tend to infinity.

Hence the statement 2 should be sufficient.

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Hi

In your example, if 3333333333 is present, then its sure that 3333333330 is also present.
and now since 3333333330 is present, we are sure that 3333333327 is also present.
Similarly 3333333324, 3333333321, .........., 9, 3, 0, -3, -6, -9, .... are ALL present.

But we cannot be sure about multiples of 3 greater than 3333333333. 3333333336 might or might not be present. Statement 2 doesn't say that 'element + 3' cannot be present, it says that 'element - 3' has to be present.

So we cannot be sure whether ALL positive multiples of 3 are present or not.

Thats why statement 2 is NOT sufficient.
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Re: If P is a set of integers and 3 is in P, is every positive [#permalink]
Hi,
But what validates statement 1 is that for every integer , t, t+3 exists in the set, and as solution says, the set is infinite. It DOES ENSURE that all positive multiples of 3 are in the set.
The question stem reads " is every positive multiple of 3 in P?" Its doesn't say all the elements of P are multiple of 3. Domain of " all possible multiples of 3" is not same as the domain of set P, instead its much larger.

I hope I am able to convey the message.

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Re: If P is a set of integers and 3 is in P, is every positive [#permalink]
Mudit27021988 wrote:
Hi,
But what validates statement 1 is that for every integer , t, t+3 exists in the set, and as solution says, the set is infinite. It DOES ENSURE that all positive multiples of 3 are in the set.
The question stem reads " is every positive multiple of 3 in P?" Its doesn't say all the elements of P are multiple of 3. Domain of " all possible multiples of 3" is not same as the domain of set P, instead its much larger.

I hope I am able to convey the message.

Posted from my mobile device

Hi

As you yourself said, "statement 1 DOES ENSURE that all positive multiples of 3 are in the set". Hence it is sufficient to answer the Question with a YES

But statement 2 just ensures that all negative multiples of 3 are in P"..because its given that 3 is in P, so 3-3=0 also must be there, so 0-3 =-3 also must be there, so -3-3= -6 also must be there and so on... But we simply cannot say whether positive multiples of 3 are present in P or not. There could be some positive multiples of 3 in P, there could be ALL positive multiples of 3 in P who knows.. we just cannot answer the asked question with either a YES or a NO.. thats why statement 2 is not sufficient.
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Re: If P is a set of integers and 3 is in P, is every positive [#permalink]
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Caffmeister wrote:
If P is a set of integers and 3 is in P, is every positive multiple of 3 in P?

(1) For any integer in P, the sum of 3 and that integer is also in P.

(2) For any integer in P, that integer minus 3 is also in P.

(1) We can take any integer of set P, 3 is in P. Then 3+3=6; 6+3= 9... and so on. Sufficient.

(2) We can take any integer of set P, 3 is in P. Then 3-3=0; 0-3=-3 and so on. So, P has (3, 0, -3). Thus P has a Positive number, non-negative, and negative. Insufficient.

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Re: If P is a set of integers and 3 is in P, is every positive [#permalink]
Regarding Statement 1, what if 1 is in P ---> 1+3=4 which is not multiple of 3 (Same goes for 2,4,5.....) =>Insufficient

Regarding statement 2, What if 10 is in P--> 10-3=7 which is not multiple of 3=>Insufficient

Taking 1 and 2 together suppose 10 is in P --> 10-3=7 and 10+3=13 in P and both 7 and 13 are not multiple of 3 =>Insufficient
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Re: If P is a set of integers and 3 is in P, is every positive [#permalink]
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