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

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

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.

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.

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%....

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.