MHB What Is the Probability of Three Girls Given the Youngest Is Female?

  • Thread starter Thread starter annie122
  • Start date Start date
  • Tags Tags
    Probabilities
AI Thread Summary
The probability of having three girls in a family of four children, given that the youngest is female, is calculated to be 0.375. This is derived from considering the possible combinations of the other three children, where the youngest is already established as a girl. The calculations show that there are three favorable outcomes (GGGB, GGBG, GBGG) out of eight total possibilities. Additionally, the discussion touches on how to determine positive correlation between two variables, although this topic is less explored. The consensus on the probability calculation provides clarity on the problem.
annie122
Messages
51
Reaction score
0
i got the answer to the following problem wrong:
"there are four children in in the family. what is the probability that there are three girls, given that the youngest child is female?"

my (updated) answer:
the youngest is female, so three out of two children must be female. there are three ways of this happening, (=3C2) so, the answer is (1/2) ^3 * 3 = .375

also, how do i determine if two variables are positively correlated?
 
Mathematics news on Phys.org
Re: two probabilities question

Yuuki said:
i got the answer to the following problem wrong:
"there are four children in in the family. what is the probability that there are three girls, given that the youngest child is female?"

my (updated) answer:
the youngest is female, so three out of two children must be female. there are three ways of this happening, (=3C2) so, the answer is (1/2) ^3 * 3 = .375

also, how do i determine if two variables are positively correlated?

Lets suppose that the probability of child male and child female is the same, i.e. $p = \frac{1}{2}$. If no information is allowable, then the probability to have three girls and one boy is... $\displaystyle P = \binom{4}{3}\ \frac{1}{16} = \frac{1}{4}\ (1)$

However if You know a priori that one is famale, the probability to have three girls and one boy is the probability to have two girls and one boy among the remaining childs and it is... $\displaystyle P = \binom {3}{2}\ \frac{1}{8} = \frac{3}{8}\ (2)$ Kind regards $\chi$ $\sigma$
 
Re: two probabilities question

Here is another way to do this: writing "G" for "girl", "B" for "boy", in order from youngest to oldest we could have
GGGG
GGGB
GGBG
GGBB
GBGG
GBGB
GBBG
GBBB
The first letter is always "G" because we are told that the youngest child is a girl. The others have 2^3= 8 possible orders giving 8 possible situations. Of those 8, exactly three have 3 "G" (GGGB,, GGBG, GBGG). Assuming that boys and girls are equally likely the probability of "three girls" is 3/8= 0.375.

(If the problem were "at least three girls" we would include "GGGG" so the probability would be 4/8= 0.5.)
 
Re: two probabilities question

thanks, I'm cleared now :)
 
Suppose ,instead of the usual x,y coordinate system with an I basis vector along the x -axis and a corresponding j basis vector along the y-axis we instead have a different pair of basis vectors ,call them e and f along their respective axes. I have seen that this is an important subject in maths My question is what physical applications does such a model apply to? I am asking here because I have devoted quite a lot of time in the past to understanding convectors and the dual...
Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. In Dirac’s Principles of Quantum Mechanics published in 1930 he introduced a “convenient notation” he referred to as a “delta function” which he treated as a continuum analog to the discrete Kronecker delta. The Kronecker delta is simply the indexed components of the identity operator in matrix algebra Source: https://www.physicsforums.com/insights/what-exactly-is-diracs-delta-function/ by...

Similar threads

Replies
2
Views
5K
Replies
2
Views
4K
Replies
3
Views
3K
Replies
4
Views
5K
Replies
12
Views
2K
Back
Top