Computing basic conditional probability. A box contains three coins: two regular coins and one fake two-headed coin. You pick a coin at random and toss it, and get heads. What is the probability that it is the two-headed coin? I hope you can help me.Thanks.

let A be the event that you choose the two-headed coin, and this is clear $P(H \mid A)=1$.And, you are interested in finding $P(A\mid H)$.By using the Bayes rule $P(A\mid H)$=$\frac{P(H \mid A).P(A)}{P(H)}=\frac{1.1/3}{2/3}=\frac{1}{2}.$