The Math Forum



Search All of the Math Forum:

Views expressed in these public forums are not endorsed by NCTM or The Math Forum.


Math Forum » Discussions » sci.math.* » sci.math

Topic: probability question
Replies: 9   Last Post: Mar 13, 2008 4:08 PM

Advanced Search

Back to Topic List Back to Topic List Jump to Tree View Jump to Tree View   Messages: [ Previous | Next ]
The World Wide Wade

Posts: 1,507
Registered: 2/24/07
Re: probability question
Posted: Mar 13, 2008 4:08 PM
  Click to see the message monospaced in plain text Plain Text   Click to reply to this topic Reply

In article
<b66d6af9-9850-477f-9e18-1892ebafdfd1@i12g2000prf.googlegroups.com>,
Ray Vickson <RGVickson@shaw.ca> wrote:

> On Mar 11, 6:56 pm, Steven <sgottlie...@hotmail.com> wrote:
> > Suppose you meet me on a street corner and I introduce you to my son who is
> > with me. I inform you that I have another child at home. What is the
> > probability that my other child is a girl.
> > I looked at the sample space which I claim is bb, bg, gb and so the answer
> > is 2/3. Is this correct?
> > Steven

>
> Yes, provided that you don't say which child is older. If you do say
> which is older, the probability becomes 1/2! You can also look at the
> problem this way: you toss a coin twice and announce that you got at
> least one Head. What is the probability the other outcome was a Tail?


"Other" is not well-defined in this coin experiment, but it is in the
OP's problem. I think your coin example models the following: You meet
me on the corner, I tell you I have two kids at home, you ask if
they're both girls, and I say no. Then the probability is 2/3 that I
have a daughter. But if I'm out with one of my two kids, who's a boy,
then the probability my other kid is a girl is 1/2 - assuming I'm
equally likely to have my other kid with me. I think this is a better
model for the OP's problem. The coin problem analogous to this is: I
have two coins, I toss each, then randomly choose the result of one of
the tosses to announce. If I say heads, what is the probability the
other toss is tails?

> Again, the probability that the other outcome is a Tail is 2/3 if you
> don't say whether your announced Head was first or second, and is 1/2
> if you do announce its position.
>
> R.G. Vickson




Point your RSS reader here for a feed of the latest messages in this topic.

[Privacy Policy] [Terms of Use]

© The Math Forum at NCTM 1994-2017. All Rights Reserved.