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How many randomly assembled people are needed to have a better than 50

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Joined: 26 Sep 2017
Posts: 22
GMAT 1: 640 Q48 V30
Re: How many randomly assembled people are needed to have a better than 50  [#permalink]

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New post 10 Apr 2018, 22:43
Bunuel wrote:
Bakshi121092 wrote:

Hi Bunuel,

What is wrong in the following approach?

Cases considered :
1. Born in a leap year
2. Not born in a leap year

In that case, the probability of not born in a leap year =1/2

And subsequently, the probability that at least one of them is born in a leap year would turn out to be greater than 50% for 2 people.

Please help.

Thank you.


Let me ask you: is the probability that you win a lottery 1/2?
1. You win the lottery
2. You won't win the lottery

Or: what is the probability to meet a dinosaur on the street? Is it 1/2? Either you meet it or not?

22. Probability



For more:
ALL YOU NEED FOR QUANT ! ! !
Ultimate GMAT Quantitative Megathread.


I understand. But the question doesn't mention a time frame.
For example, considering the probability to be 3/4 is for any 4 year time frame. If we consider a 5 year time frame, the probabilities change.

The probabilities you asked depend on other factors. One can simply not tell. But with no other information given, the probability that I could meet a dinosaur would be 1/2. I may or may not encounter a dinosaur.

Please help.
Thanks.
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Joined: 02 Sep 2009
Posts: 54696
Re: How many randomly assembled people are needed to have a better than 50  [#permalink]

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New post 10 Apr 2018, 23:35
Bakshi121092 wrote:
Bunuel wrote:
Bakshi121092 wrote:

Hi Bunuel,

What is wrong in the following approach?

Cases considered :
1. Born in a leap year
2. Not born in a leap year

In that case, the probability of not born in a leap year =1/2

And subsequently, the probability that at least one of them is born in a leap year would turn out to be greater than 50% for 2 people.

Please help.

Thank you.


Let me ask you: is the probability that you win a lottery 1/2?
1. You win the lottery
2. You won't win the lottery

Or: what is the probability to meet a dinosaur on the street? Is it 1/2? Either you meet it or not?

22. Probability



For more:
ALL YOU NEED FOR QUANT ! ! !
Ultimate GMAT Quantitative Megathread.


I understand. But the question doesn't mention a time frame.
For example, considering the probability to be 3/4 is for any 4 year time frame. If we consider a 5 year time frame, the probabilities change.

The probabilities you asked depend on other factors. One can simply not tell. But with no other information given, the probability that I could meet a dinosaur would be 1/2. I may or may not encounter a dinosaur.

Please help.
Thanks.


That's not correct.

There is one leap year in four. So, the probability of random person being born in leap year is 1/4. As for the dinosaur problem, no other information is needed apart from common knowledge, the probability is not 1/2: meeting and not meeting are not equally likely, and simply the fact that there are only two options does not mean that the provability is 1/2.

I suggest you to follow the links given in my previous post and maybe read some simple book on probability (introduction of some kind).
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Re: How many randomly assembled people are needed to have a better than 50  [#permalink]

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New post 22 Mar 2019, 01:31
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Re: How many randomly assembled people are needed to have a better than 50   [#permalink] 22 Mar 2019, 01:31

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