Author Topic: Well-known, but don't look up  (Read 4685 times)

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Offline Ender

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Well-known, but don't look up
« on: March 15, 2008, 12:04:05 am »
A random lady has two children, and at least one of them is a boy. What are the chances that the other one is a boy?

Offline rabbit

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Re: Well-known, but don't look up
« Reply #1 on: March 15, 2008, 07:58:34 am »
I remember doing this in high school, but I don't remember how.....or what my answer was......dammit.

Offline iago

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Re: Well-known, but don't look up
« Reply #2 on: March 15, 2008, 10:49:44 am »
A random lady has two children, and at least one of them is a boy. What are the chances that the other one is a boy?
Well, it depends on genetics. Are some people more pre-disposed to have boys or girls? Could having one boy indicate that mother's pre-disposition, thus making it slightly more likely for her to have another boy?

In any case, I'd say the chances are approximately 50%, unless there's some tricky wording.

(I say approximately because of this)

Offline Towelie

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Re: Well-known, but don't look up
« Reply #3 on: March 15, 2008, 11:03:54 am »
Are you asking whats the probability to have two boys in a row? Or just the probability that the next one is a boy? 25% chance that she has two boys in a row, 50% chance that the 2nd one is a boy.

Offline iago

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Re: Well-known, but don't look up
« Reply #4 on: March 15, 2008, 12:03:57 pm »
Are you asking whats the probability to have two boys in a row? Or just the probability that the next one is a boy? 25% chance that she has two boys in a row, 50% chance that the 2nd one is a boy.
Good point, but I *think* that the first boy is assumed, by the wording of the question.

Offline Dale

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Re: Well-known, but don't look up
« Reply #5 on: March 15, 2008, 03:44:32 pm »
I agree with you iago, but I also was curious if the word "random" put in there, had something to do with tricky wording.
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Offline Ender

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Re: Well-known, but don't look up
« Reply #6 on: March 15, 2008, 03:58:44 pm »
No one has gotten the right answer yet ;). And yes, we are assuming that a boy is just as likely to be born as a girl.

Offline iago

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Re: Well-known, but don't look up
« Reply #7 on: March 15, 2008, 04:05:37 pm »
I was just reading this again, and it suddenly reminded me of the three-door paradox. There's a prize behind one door. Pick one. You picked two? Well, let's open three. No prize? That's good. Now, do you want to open the one you picked or the other?

(answer is the other, as long as the other person's selection was random, because the odds are now 50/50 for the other, compared to the 33/66 that you picked the first one with)

So what it comes down to is sort of paradoxical. There are two distinct scenarios, and it does hinge on the word "random" (thanks, Dale!)

If you take a random person with 2 children, there are 4 combinations:
Boy/Boy
Boy/Girl
Girl/Boy
Girl/Girl

For the purposes of this question, we choose a random family. That family has one child who's a boy. That means that it's one of these:
Boy/Boy
Boy/Girl
Girl/Boy

So if one child is a boy, what is the other? 1/3 of the time, it's a boy, and 2/3 of the time, it's a girl.

I don't really understand how it works, the same way I don't really understand how that door problem works, but all I know is, it makes my head hurt trying to think about it. :)

Offline topaz~

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Re: Well-known, but don't look up
« Reply #8 on: March 15, 2008, 04:07:52 pm »
A random lady has two children, and at least one of them is a boy. What are the chances that the other one is a boy?
100%?

Never mind, I reread the question again. I thought it was a riddle sort thing where the answer is readily available to you.
« Last Edit: March 15, 2008, 04:09:52 pm by igimo »

Offline Ender

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Re: Well-known, but don't look up
« Reply #9 on: March 15, 2008, 04:08:58 pm »
Yep. 1/3. Correct, iago!

Offline Sidoh

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Offline Ender

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Re: Well-known, but don't look up
« Reply #11 on: March 15, 2008, 04:14:54 pm »
A random lady has two children, and at least one of them is a boy. What are the chances that the other one is a boy?
100%?

Never mind, I reread the question again. I thought it was a riddle sort thing where the answer is readily available to you.

Heh. The wording is tricky, but I made it essentially same as the original wording, taking out the fluff at the start. The word "other" doesn't mean the one who's not a boy. We know that there's one child who is certainly a boy, and one child of uncertain gender, though we don't know which has certain gender and which has uncertain gender. The word "other" refers to the child of uncertain gender.

Offline Ender

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Re: Well-known, but don't look up
« Reply #12 on: March 15, 2008, 04:17:13 pm »
Yeah, this was a question that a reader asked Marilyn Vos Savant. I think it's called the "Two boys" problem.

Offline Dale

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Re: Well-known, but don't look up
« Reply #13 on: March 15, 2008, 04:23:26 pm »
I was just reading this again, and it suddenly reminded me of the three-door paradox. There's a prize behind one door. Pick one. You picked two? Well, let's open three. No prize? That's good. Now, do you want to open the one you picked or the other?

(answer is the other, as long as the other person's selection was random, because the odds are now 50/50 for the other, compared to the 33/66 that you picked the first one with)

So what it comes down to is sort of paradoxical. There are two distinct scenarios, and it does hinge on the word "random" (thanks, Dale!)

If you take a random person with 2 children, there are 4 combinations:
Boy/Boy
Boy/Girl
Girl/Boy
Girl/Girl

For the purposes of this question, we choose a random family. That family has one child who's a boy. That means that it's one of these:
Boy/Boy
Boy/Girl
Girl/Boy

So if one child is a boy, what is the other? 1/3 of the time, it's a boy, and 2/3 of the time, it's a girl.

I don't really understand how it works, the same way I don't really understand how that door problem works, but all I know is, it makes my head hurt trying to think about it. :)


Good job, the "random" just stuck out of context to me for some odd reason.
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[1:53:54 PM] Ersan: i could've written iago's code in 20 minutes
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Kitty: Well honey, Canadians don't matter.
Dude, peanutbutter and jelly is like...1000000x more intense than a Volvo.

Offline Ender

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Re: Well-known, but don't look up
« Reply #14 on: March 15, 2008, 04:26:32 pm »
I didn't need to use the word "random," but I just wanted to emphasize that this lady had no proclivity for giving birth to boys over girls.