Sue has two coins. One is fair, with a head on one face and a tail on the other. The second is a trick coin and has a tail on both faces. Sue picks up one of the coins at random and flips it.
a) Find the probability that it lands heads up.
b) Given that it lands tails up , find the probability that she picked up the fair coin.
My turn:
a) We have one head out of three tails and one head, so the answer is $\frac{1}{4}$.
b) I do not understand how can i start with this?!
Best Answer
Another way: Among the four equally likely outcomes, three are favorable to "tails" and one of those three is favorable to "fair". Hence $\Pr(\text{fair}\mid\text{tails}) = 1/3.$