Solution:

Let represent the event of selecting a two-headed coin, represent the event of selecting a biassed coin, and represent the event of selecting an unbiased coin. Let be the event in which the coin lands on its head.

Then

Because a headed coin has a head on both sides, it will display one.

Also (correct answer given that he knows)

And (coin shows head given that the coin is biased) And (coin shows head given that the coin is unbiased)

Given that the coin is showing head, the probability that it is two-headed is .

We have used Bayes’ theorem to arrive at our conclusion.

We get the result by swapping the values.