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## #1 2017-10-12 05:25:09

Abbas0000
Member
Registered: 2017-03-18
Posts: 29

### Probabilty of a family's children

You have 3 children in a family . What is the probability of having 2 girls ?
I've confuesd . I've tryed this method but I don't know why it is not working!
First for n(s) we can say 2*2*2 (every child have two possibility)so it equals 8 then for n(a) we can say because we want 2 girls so 2 children would have just one possibility namely , 1*1*2 then it would equal to 2. But if you write down every possibilty it would be 3! My approach is the same thing for calculating sub sets of some (X) set that is n(s), and for n(A) as I've mentioned . So if ANYONE knows what's wrong or know a better way than write down all possibilties you can make me delighted by leaving it in the comment! THANK YOU;-)

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## #2 2017-10-12 23:34:54

bob bundy
Registered: 2010-06-20
Posts: 8,442

### Re: Probabilty of a family's children

hi Abbas0000

You are correct that there are 8 possibilities:

BBB
BBG
BGB
BGG
GBB
GBG
GGB
GGG

We'll assume boy or girl is equally likely. (in fact it's not quite 50:50 but pretty close).

Each possibility has a probability of 1/8

Three of those listed have exactly two girls so P(two girls) = 1/8 + 1/8 + 1/8 = 3/8

There a formula for this:

p is the probability of an event; q = (1-p) is the probability of the event not happening; n is the total number of events; r is the number that we want.

You can find more on this here:

http://www.mathsisfun.com/data/binomial … ution.html

Bob

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob Bundy

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## #3 2017-10-13 06:14:37

Abbas0000
Member
Registered: 2017-03-18
Posts: 29

### Re: Probabilty of a family's children

THANK YOU SO MUCH!! I were struggling to understand this (binomial page really worked)

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