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**math951** Identical twins come from the same egg and hence are the same sex. Fraternal Twins have a 50-50 chance of being the same sex. Among twins, the probability of a fraternal set is p, and an identical set is q=1-p. If the next set of twins are the same sex, what is the probability that they are identical?

I will denote B as the event of the next set of twins are the same sex. I will then denote A as the event of the next set of twins are identical.

So we know that if they are the same sex they are either identical or fraternal.

So then we know that P[A]= q , therefore, P[A']= p= q-1

We want to find P[ A | B ] using Bayes theorem.