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This DS questions is a great example of how you can "rewrite" the question to find "the question behind the question."

We're told that an unknown number of people EACH wrote down one of the first 30 positive integers (1-30, inclusive). We're asked if ANY of the integers were written down by MORE than one person. This is a YES/NO question.

Given the 'restrictions' in this question, IF there are MORE than 30 people, then at least one of the numbers would be repeated. If there are 30 or LESS, then it's possible that a number was repeated, BUT it's also possible that NONE of the numbers were repeated. So the question is really asking...."Were there more than 30 people?"

Fact 1: The number of people....was greater than 40

This GUARANTEES that at least 1 of the numbers was written more than once, so the answer to the question is ALWAYS YES. Fact 1 is SUFFICIENT

Fact 2: The number of people....was less than 70

IF.... The total number of people was greater than 30, then the answer to the question is YES.

IF... The total number of people is 30 or LESS, then the answer to the question COULD be YES, but it COULD be NO. Fact 2 is INSUFFICIENT.

A number of people each wrote down one of the first 30 positive integers. Were any of the integers written down by more than one of the people?

There are 30 integers from 1 to 30, inclusive, thus the maximum number of people possible all of them to write different integers is 30. If there are more than 30 people, then at least one of the integers has to be written by more than one person.

(1) The number of people who wrote down an integer was greater than 40 --> at least one of the integers has to be written by more than one person. Sufficient. (2) The number of people who wrote down an integer was less than 70. Not sufficient.

Re: A number of people each wrote down one of the first 30 [#permalink]

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07 May 2013, 00:18

Bunuel wrote:

A number of people each wrote down one of the first 30 positive integers. Were any of the integers written down by more than one of the people?

There are 30 integers from 1 to 30, inclusive, thus the maximum number of people possible all of them to write different integers is 30. If there are more than 30 people, then at least one of the integers has to be written by more than one person.

(1) The number of people who wrote down an integer was greater than 40 --> at least one of the integers has to be written by more than one person. Sufficient. (2) The number of people who wrote down an integer was less than 70. Not sufficient.

Answer: A.

How can we say that for sure? Its also possible that all the individuals wrote down 1. There is no mention that everyone has to write a unique number.

A number of people each wrote down one of the first 30 positive integers. Were any of the integers written down by more than one of the people?

There are 30 integers from 1 to 30, inclusive, thus the maximum number of people possible all of them to write different integers is 30. If there are more than 30 people, then at least one of the integers has to be written by more than one person.

(1) The number of people who wrote down an integer was greater than 40 --> at least one of the integers has to be written by more than one person. Sufficient. (2) The number of people who wrote down an integer was less than 70. Not sufficient.

Answer: A.

How can we say that for sure? Its also possible that all the individuals wrote down 1. There is no mention that everyone has to write a unique number.

Say the number of people is 41. Each should write down one of the first 30 positive integers (1, 2, 3, ..., 30). Ask yourself, can each of them write the different integer?
_________________

Re: A number of people each wrote down one of the first 30 [#permalink]

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07 May 2013, 01:56

A number of people each wrote down one of the first 30 positive integers. Were any of the integers written down by more than one of the people?

(1) The number of people who wrote down an integer was greater than 40. (2) The number of people who wrote down an integer was less than 70.

I am not sure what is stopping all 40 people from writing down number 1 on their paper? Why does it has to be a sequence.. All of the 40 can just write a single number (i.e. 1) on their paper.

A number of people each wrote down one of the first 30 positive integers. Were any of the integers written down by more than one of the people?

(1) The number of people who wrote down an integer was greater than 40. (2) The number of people who wrote down an integer was less than 70.

I am not sure what is stopping all 40 people from writing down number 1 on their paper? Why does it has to be a sequence.. All of the 40 can just write a single number (i.e. 1) on their paper.

Yes, and in this case one of the integers (1) will be written down by more than one of the people. Thus the answer to the question will be YES. Does this make sense?
_________________

Re: A number of people each wrote down one of the first 30 [#permalink]

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07 May 2013, 02:25

Ah....I get the problem now. Its an easy problem! Actually I was thinking exactly the opposite in my brain and wondering why am i getting the wrong answer!

Time to cut down on careless mistakes

Thanks for your patience while my brain was switched off

Re: A number of people each wrote down one of the first 30 [#permalink]

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02 Aug 2014, 16:48

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A number of people each wrote down one of the first 30 [#permalink]

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05 Dec 2014, 16:36

This one can be confusing... I made the mistake of not reading the statements properly. The statements are giving the number of people not the sum of the amount of the number that the people wrote down.

Statement 1: There at least 10 numbers that are written twice because 30 of the people could write down different numbers but after that the rest of the people have to reuse numbers that were already written down.

Statement 2: The number of people could be 30 and each one writing a different number... So we don't know if there are any duplicate numbers.

Re: A number of people each wrote down one of the first 30 [#permalink]

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26 Apr 2015, 13:00

Hi,

Consider there are 3 ppl and we are asked to write down any integers from 1 to 30. I could write 2 the next one could write 10 ,the 3rd one could write 2 as well.So does the number of ppl really matter here.

By the above logic both the statements have to be correct.

A number of people each wrote down one of the first 30 [#permalink]

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26 Apr 2015, 13:10

kirtivardhan wrote:

Hi,

Consider there are 3 ppl and we are asked to write down any integers from 1 to 30. I could write 2 the next one could write 10 ,the 3rd one could write 2 as well.So does the number of ppl really matter here.

By the above logic both the statements have to be correct.

Please correct me if i am wrong

In DS questions, statement will be sufficient if answer is definite. In case which you descrive two variants are possbile: Some of this 3 persons could write the same numbers or maybe they are wrote three different numbers. For DS this is insufficient answer.
_________________

Re: A number of people each wrote down one of the first 30 [#permalink]

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26 Apr 2015, 13:52

kirtivardhan wrote:

Following the same logic,both the statements should have been insufficient right?

No, because from first statement you know that numbers will be repeat in ANY case. Because 40 people can't write first 30 numbers and don't repeat them.

So it's definite answer and this sufficient.
_________________

Re: A number of people each wrote down one of the first 30 [#permalink]

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28 Apr 2015, 20:26

Hi Bunnel,

The thing i am not able to understand is why not 2 ppl write the same number.In that case whether the number of ppl >40 or <70 the number that they write will be repeated.

Re: A number of people each wrote down one of the first 30 [#permalink]

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28 Apr 2015, 23:35

kirtivardhan wrote:

Hi Bunnel,

The thing i am not able to understand is why not 2 ppl write the same number.In that case whether the number of ppl >40 or <70 the number that they write will be repeated.

You understand absolutely right: people from 2 statement will write the same number in any case. And we sure about this and that's why second statement Sufficient.
_________________

The thing i am not able to understand is why not 2 ppl write the same number.In that case whether the number of ppl >40 or <70 the number that they write will be repeated.

Please assist

Hi kirtivardhan,

The 'tricky' part about this question is that we don't know what number each individual person wrote down.

As an example....If there were only 2 people, it's POSSIBLE that they both wrote down the SAME number, but it's ALSO POSSIBLE that they wrote down two DIFFERENT numbers.

In Fact 2, we're told that there are fewer than 70 people....so there COULD be just 2 people and the above 2 results are both possible. That's why Fact 2 is INSUFFICIENT.

Re: A number of people each wrote down one of the first 30 [#permalink]

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23 Jul 2016, 05:38

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
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