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Fraternal Twins

rstrats

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A woman is about to give birth to fraternal twins. What is the chance that one of the twins will be a girl?​
 

Deacon

Well-Known Member
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Chances

Girl - Girl
Girl - Boy
Boy - Girl
Boy - Boy


Odds are 75% at least one will be a girl.

Rob
 

Don

Well-Known Member
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Although, I do accept that it also depends on which came out first, so I understand 75%....
 

Deacon

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Now I know Dr. Cassidy as a very precise person on the BB - he reads things exactly as they are written.

The question as raised in the OP was, "What is the chance that one of the twins will be a girl?"

As he noted, there is a 50% chance that only one will be a girl.

Rob
 

rstrats

Member
Site Supporter
Deacon,

re: "Odds are 75% at least one will be a girl."


And the interesting thing is that there is also a 75% chance that one of the twins will be a boy.
 

Don

Well-Known Member
Site Supporter
Now I know Dr. Cassidy as a very precise person on the BB - he reads things exactly as they are written.

The question as raised in the OP was, "What is the chance that one of the twins will be a girl?"

As he noted, there is a 50% chance that only one will be a girl.

Rob

With al due respect to Dr. Cassidy and you -- incorrect. First, the opening post did not specify "only" one. Second, in accordance with the wording of the opening post, girl/girl is a valid option, raising the odds to 66% instead of 50%. If the opening post had specified "only" one is a girl, then the options would be limited to boy/boy and boy/girl, resulting in 50%. If the opening post had specified the birth order, then Deacon's 75% would be valid. But a strict reading of the opening post leaves only the 2 out of 3 option.
 

Don

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Well...my apologies. The question is more complex than I originally thought.

On the one hand, with twin comparisons, the answer is 66%. However, if one starts using truth tables, you get:
B G
B F T
G T T
(B-boy, G-girl, T-true, F-false)
And thus, 75%.

Alternatively, upon birth of the first child, if it's a boy, there's a 50% chance the next child will be a girl. If the first child is a girl, then we've reached 100% probability. The average of these two alternatives is also 75%.

You could make it even more difficult, and throw in the probability that 40% of twins are boy/girl, leaving 30% equally for boy/boy and girl/girl (except that statistically, boy/boy twins miscarry more than girl/girl, so that actually increases the odds of a girl child....)

In the end analysis, I should probably apologize to Deacon and go with the 75%....
 

rstrats

Member
Site Supporter
Don,

re: "In the end analysis, I should ...go with the 75%...."


That is correct. As you noted, there are 4 possible combinations and a girl shows up in 3 of them.
 
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