Black or White 3
John provided the following problem on a comment on the “Black or White” page:
There are a number of cards in a hat, 20% of which are white on both sides, 50% of which are black on one side and white on the other and 30% of which are black on both sides.
One card is drawn and only one side is inspected. If that side is black, what is the probability that the other side is white?