Brain teaser

Burnsy2023 · 29 Nov 2011 at 15:47

You meet a man on the street and he says, “I have two children and one is a son born on a Tuesday.” What is the probability that the other child is also a son?

loltim · 29 Nov 2011 at 15:50

lookitsjonno · 29 Nov 2011 at 15:51

Pottsey · 29 Nov 2011 at 15:52

50%

EDIT: The other child is either male or female so 50/50 or 50% unless you want to start factoring in chances of twins, chances of chromosomes being female or male and then it gets complicated.
EDIT2: Assuming an equal chance of a birth being male or female (not strickly speaking 100% true). Having a child is like rolling a dice, there is no memory. Once you role a dice and get a 6 then 2nd role is a 1 in 6 chance to get another 6. Same for a birth once the first has happened there is no memory and it’s a 50/50 what the 2nd child will be.

Mr Jack · 29 Nov 2011 at 15:57

One third.

A.N.Other · 29 Nov 2011 at 15:58

Don't really care, but I'd steer well clear of him in the future

Cosimo · 29 Nov 2011 at 16:00

loltim · 29 Nov 2011 at 16:01

wait, my head hurts.

:confused:

Ascendancy · 29 Nov 2011 at 16:02

This isn't a brain teaser it's a famous paradox lol.

Nate--IRL-- · 29 Nov 2011 at 16:02

Good question and it lead to some interesting reading that gave me a headache.

Nate

Davefran · 29 Nov 2011 at 16:03

13/27

Xordium · 29 Nov 2011 at 16:03

13/27

Maccy · 29 Nov 2011 at 16:04

A third (33.333333%)

Pottsey · 29 Nov 2011 at 16:06

How are some of you getting odd numbers like 1/3. 100% or 0%?

Elixir · 29 Nov 2011 at 16:11

Pottsey said:
How are some of you getting odd numbers like 1/3. 100% or 0%?

http://en.wikipedia.org/wiki/Boy_or_Girl_paradox

lookitsjonno · 29 Nov 2011 at 16:12

it's how its worded - why would someone say "one is a son born on a tuesday" if the other was also a son? If his other was also a son he would say "I have two sons, one born on a tuesday, the other on a friday".

bah thats my guess anyway.

Elixir · 29 Nov 2011 at 16:15

lookitsjonno said:
it's how its worded - why would someone say "one is a son born on a tuesday" if the other was also a son? If his other was also a son he would say "I have two sons, one born on a tuesday, the other on a friday".

bah thats my guess anyway.

No, it's just down to probability.

There are four possible combinations of children:
Girl/Girl
Girl/Boy
Boy/Girl
Boy/Boy

If you say one of them is a boy, it leaves three possible situations, one of which results in two boys. Therefore 1/3 chance of there being two boys.

The Tuesday bit is what makes it 13/27, but that's more complex and I can't be bothered typing it.

Google if interested.

Pottsey · 29 Nov 2011 at 16:24

Elixir said:
No, it's just down to probability.

There are four possible combinations of children:
Girl/Girl
Girl/Boy
Boy/Girl
Boy/Boy

If you say one of them is a boy, it leaves three possible situations, one of which results in two boys. Therefore 1/3 chance of there being two boys.

The Tuesday bit is what makes it 13/27, but that's more complex and I can't be bothered typing it. Google if interested.

I didn't think probability work like that. When you flip a coin and get a head you don't say the 2nd flip has a 1/3 chance to be a head again. You say per flip 50/50 chance. To me saying 1/3 is like saying you have a 1/3 chance to get heads on a coin toss.

When you have a child the chance is around 50% to be a boy or girl. Once the first child is born it no longer factors in and the 2nd child has a 50% chance to be a boy or girl. So knowing the sex of the first child has no impact on knowing the sex of the 2nd child.

Knowing the previous result of a 50/50 result has zero impact on the 2nd result. So the 2nd result stays 50/50.

crinkleshoes · 29 Nov 2011 at 16:24

Ascendancy said:
This isn't a brain teaser it's a famous paradox lol.

Try looking up the definition of a paradox - it isn't one

It's quite simple really... depending on how it's written it's likely to be interpretted in different ways, but the answer is always the same - 50%

If written like this:

I have two children and one is a son, born on a Tuesday

Then people would tend to get the impression that as one is a son, the other is a daughter.

However, there is no certainty expressed anywhere in the statement, with no clue as to any details about the other child.

So... the answer is always 50%

crinkleshoes · 29 Nov 2011 at 16:26

Elixir said:
No, it's just down to probability.

There are four possible combinations of children:
Girl/Girl
Girl/Boy
Boy/Girl
Boy/Boy

If you say one of them is a boy, it leaves three possible situations, one of which results in two boys. Therefore 1/3 chance of there being two boys.

The Tuesday bit is what makes it 13/27, but that's more complex and I can't be bothered typing it. Google if interested.

Wrong...

The chance of the other child being a boy or a girl is independant of the first child, the possible combinations is a seperate question.