From four tabby and five black kittens two are chosen without replacement. What is the probability that both are the same colour?

1. Two tabby kittens arrive with probability 4/9 times 3/8, which is 12/72. 2. Two black kittens arrive with probability 5/9 times 4/8, which is 20/72. 3. Those two ways are mutually exclusive, so their probabilities add. 4. Adding gives 12/72 plus 20/72, which is 32/72. 5. Stopping at 12/72 counts only the tabby pair and forgets the black one. 6. The value 40/72 is the probability of one kitten of each colour, which is the complement of this event. _Source: OpenStax Introductory Business Statistics (CC BY 4.0), Ch 3 "Probability Topics", section 3.4 Contingency Tables and Probability Trees_