Teaser Time

[DixieDean]

seeing as we're a little short of topics at the moment I thought I'd get everyone's brain going with this teaser....

A man has two children. One of them is a boy. What's the probability that the other one is a boy?

 

[championship_here_we_come]

seeing as we're a little short of topics at the moment I thought I'd get everyone's brain going with this teaser....

A man has two children. One of them is a boy. What's the probability that the other one is a boy?

2/4=1/2

 

[DixieDean]

no sorry.
most people go for the 50-50 option but it aint right i'm afraid.

 

[championship_here_we_come]

1/3

 

[smilingmart]

As much chance as eric nixon being a successful manager. :wink:

 

[DixieDean]

1/3

yes....can you explain why?

 

[NeverBeatDaveHiggins]

I'm gonna stick with 50-50, since the gender of the second child is not conditional on the gender of the first. It's always luck of the draw which sperm makes it.

 

[championship_here_we_come]

1/3

yes....can you explain why?

male female
xy.......xx

x..y......x..x

xx xy xy xx

one xy has already been taken by the 1st son so in theory there is now a 1 in 3 chance of the next child being a boy

 

[DixieDean]

I'm gonna stick with 50-50, since the gender of the second child is not conditional on the gender of the first. It's always luck of the draw which sperm makes it.

If a man has two children, he could have had:

first a boy, then a girl - BG
first a boy, then another boy - BB
first a girl, then a boy - GB
first a girl, then another girl - GG

These are the only possible ways in which a man can have had 2 children, and they are all equally probable.

We know that at least one of the children is a boy. That cuts out just one of the 4 possibilities listed above - GG - leaving 3 possibilities: BG, GB, BB (each of which, as we've seen, is equally likely).

Of those 3, only one - BB - satisfies the condition that the other child is a boy as well. So the answer is 1 in 3, or 33.3% (.3 recurring to be precise).

So well done CHWC, you kinda got there!

 

[Johnny King is God]

Something about algebra makes my stomach turn. Thought I'd seen the last of that.

 

[championship_here_we_come]

I'm gonna stick with 50-50, since the gender of the second child is not conditional on the gender of the first. It's always luck of the draw which sperm makes it.

If a man has two children, he could have had:

first a boy, then a girl - BG
first a boy, then another boy - BB
first a girl, then a boy - GB
first a girl, then another girl - GG

These are the only possible ways in which a man can have had 2 children, and they are all equally probable.

We know that at least one of the children is a boy. That cuts out just one of the 4 possibilities listed above - GG - leaving 3 possibilities: BG, GB, BB (each of which, as we've seen, is equally likely).

Of those 3, only one - BB - satisfies the condition that the other child is a boy as well. So the answer is 1 in 3, or 33.3% (.3 recurring to be precise).

So well done CHWC, you kinda got there!

i actually got there by using the proper method i learnt in biology using the x and y chromosomes u have basiclly jusat re-labeled the chromosomes B and G.

the male has chromosmes of x and y the female has chromsomes of x and x

the y chromosome is dominant and is also the male chromosome

so any offspring which contain the y chromosme would be male and there are only four combinations of the chromosomes conbining these are

as the female has two x chromosomes we will call them x1 and x2

x(male) and x1(female) xx girl

y(male) and x1(female) xy boy

y(male) and x2(female) xy boy

x(male) and x2(female) xx girl


so if they have 1 boy that just leaves

x(male) and x1(female) xx girl

y(male) and x2(female) xy boy

x(male) and x2(female) xx girl

so now the probability of the second child in theory being a boy is 1/3

is theory 1/3 is correct however in practice we all know this is incorrect as some families will have all boys and no girls or the other way round

so in practice it is still 50/50 as i orginally siad and so did NeverBeatDaveHiggins

 

[Ian]

i'm feeling dizzy :-#

 

[DixieDean]

is theory 1/3 is correct however in practice we all know this is incorrect as some families will have all boys and no girls or the other way round

so in practice it is still 50/50 as i orginally siad and so did NeverBeatDaveHiggins

well you haven't quite got the point but it's far too late at night to be explaining it now.... but it is 1 in 3 not 50/50

 

[NeverBeatDaveHiggins]

Ah so the phrasing of the question is the real question. I'm right when talking about a second child yet to be born, you're right in the very specific circumstance of talking about two children already born and we're told the gender of one of them (NOT the first one, just one of them), but not the gender of the other.

 

[DixieDean]

Ah so the phrasing of the question is the real question. I'm right when talking about a second child yet to be born, you're right in the very specific circumstance of talking about two children already born and we're told the gender of one of them (NOT the first one, just one of them), but not the gender of the other.

good man. spot on

 

[bolton_hater]

this thread gave me a headache

but after realising I get it now lol

 

[Joe_TRFC]

I get it now... but never would have known if the answer hadn't been posted! Science was never my strong point... especially biology!

 

[Matthew The Rover]

I'm gonna stick with 50-50, since the gender of the second child is not conditional on the gender of the first. It's always luck of the draw which sperm makes it.

Your right as I believe I did this in Biology just last week :lol: