Elvis had a twin brother (who died at birth).
Historically, approximately $1/125$ of all births were fraternal twins and 1/300 were identical twins. The probability that Elvis was an identical twin is approximately . . .
I'm trying to understand how to apply Bayes Theorem. To try to solve this problem above, I tried to apply Bayes Theorem by plugging in values.
Let $I$ be: Being an identical twin
Let $B$ be: Having an identical twin brother
We want to calculate $P(I|B)$.
Using Bayes, $P(I|B)=P(I)*P(B|I)/P(B)$
$P(B|I)=1$ because we know that Elvis is male, so the chance of having an twin brother if we assume Elvis is an identical twin is $100\%$
How do I calculate what the chance of having an twin brother is generally? It seems like I can't apply Bayes theorem if I can't calculate this value.