I think I got this one but I want to be extra sure
Suppose we have three cards. The first one is blank on both sides, the second has an X on one side and is blank on the other side, and the third has an X on both sides. We run an experiment where we choose one card at random and then look at one side of the chosen card at random.

a. What is the probability that you see an X?

I find P(X on 1st side) = 1/2 (since 3 of the 6 sides have an X)

b. What is the probability that you see an X and the other side has an X too?

I find P(X on 1st and X on 2nd side) = 1/3 (only 1 of 3 cards has 2 X's)

c. What is the probability of X on 2nd side given X on 1st side?

I find P(X2 | X1) = (1/3) / (1/2) = 1/6