I can't obtain the answer (5/6) to this question.

Four species of fish--BM, G, GF, and N-- are available in a fish store:. A child has been told that she may choose any two of these fish species for her aquarium. Then several fish of each species will be purchased. Suppose that she is equally likely to chooses each species.

She has either a BM or a N but not both.

First pick is 1/2 and the second is 2/3 but this isn't right.