Probability with independent outcomes

[pace]

Jeeserssweesers(for a lack of a better word), probability is confusing.

Homework Statement

A family has three children, no twins or.. that same just with three.
a| What is the chance that all the children are girls.

Homework Equations

The chapter here shows the equtations P(a)+P(b)=P(a)xP(b), but my problem here maybe is that I don't find the use of that in this question.

The Attempt at a Solution

This questions looks just like the same to me when you throw three coins and ask what the chance is for that you get three on one side(?). So I get 2(boy or girl)x2x2= 8 possible outcomes. And getting three on one side is one out of these 8 outcomes, same with getting three girls. So that gives: 1/8. (=0.125). But in my book the result shows 0.115.

 

[HallsofIvy]

Yes, there are 8 possible outcomes: BBB, BBG, BGB, GBB, BGG, GBG, GGB, and GGG. 1 out of those 8 is GGG so the probability of "all girls" is 1/8= 0.125 as you say. I suspect that "0.115" is a typo.

 

[pace]

Yeah, I guessed so too, but then I've thought so before too and being wrong, so.. *g* Hm, I'll ask about b| soon.

 

[pace]

Yeah, b| the probability of that the eldest is a brother, and the two youngest girls. That is also just one outcomes yes? So that should give 1/8. But the answer writes as 0.121, which is not 1/8. ?

 

[HallsofIvy]

Yes, that would be specifically "GGB" and the probability is (1/2)(1/2)(1/2)= 1/8= 0.125. Surely they don't have another typo? This is beginning to look like shoddy editing!

 

[pace]

uh, are you joking with me, or am I writing down the question wrong(cause I know I have a bad habit of not reading the question at probability well enough)? Cause that was the way I was thinking as you above.