Two marbles are to be picked from an urn with B>0 blue and R>0 red marbles without replacement.

The probability both picked marbles are the same color is exactly 1/2. What is can be said about B and R?

Suppose, in addition to the blue and red marbles there is also a single green marble.

The probability both picked marbles are the same color is exactly 1/2. What is can be said about B and R?