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#1 2020-03-25 15:04:01

Fruityloop
Member
Registered: 2009-05-18
Posts: 139

Probability of exhuming a female.

This is from a book.  There are 2 graves.  You know that at least one of the graves contains the body of a woman.  If you randomly select one of the graves and exhume the body, what is the probability that it will be a woman?  The book gives the probability as 3/4.  I think that it is 2/3.  Thoughts?

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#2 2020-03-25 15:30:10

renevanderlende
Member
From: The Netherlands
Registered: 2020-01-06
Posts: 17

Re: Probability of exhuming a female.

Purely on intuition: the chance that you'll find a woman is twice the probability you find a man: two graves => might be 2 women. So 2 women and 1 man would yield 2/3 chance you will find a woman. This is because you know that there will at least be a woman in one of the graves. Without that knowledge it would be 50/50.

Shred that book...

Last edited by renevanderlende (2020-03-25 15:40:29)

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#3 2020-03-25 20:14:21

bob bundy
Administrator
Registered: 2010-06-20
Posts: 8,769

Re: Probability of exhuming a female.

hi Fruityloop

There's a useful tool that will help here.  It's called a tree diagram.

CBXeJM2.gif

So don't throw the book yet.  But there's plenty of ways of setting up this problem without exhuming graves.  I think the author needs psychiatric  help.

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 smile

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#4 2020-03-26 01:28:22

renevanderlende
Member
From: The Netherlands
Registered: 2020-01-06
Posts: 17

Re: Probability of exhuming a female.

Wadda ya know, scored myself another F.  Hope you've got glue, Fruityloop....

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#5 2020-03-26 05:33:24

Fruityloop
Member
Registered: 2009-05-18
Posts: 139

Re: Probability of exhuming a female.

I was thinking it was 2/3 because there are 3 equal possibilities...
Man in 1st grave.  Woman in 2nd grave.
Woman in 1st grave.  Man in 2nd grave.
Woman in 1st grave.  Woman in 2nd grave.
So the probability should be 1/3*1 + 1/3*1/2 + 1/3*1/2 = 2/3.

It seems that the author thinks that there are just 2 equal possibilities...
Woman in one grave.  Man or woman in the other grave.
That would give...
1/2*1 + 1/2*1/2 = 3/4.

This is from a story.  That's why it's a bit bizarre.

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