A certain music store stocks 800 cellos and 600 violas. Of these instruments, there are 90 cello-viola pairs such that a cello and a viola were both made with wood from the same tree (each tree can make at most one viola and one cello, so there are no pairs other than these 90). If one viola and one cello are chosen at random, what is the probability that the two instruments are made with wood from the same tree?