The theoretical probability is defined as the ratio of the number of favourable outcomes to the number of possible outcomes.
We have only two black eights on the deck and two black kings on the deck, therefore, the amount of favourable outcomes is equal to their sum, which is 4. The total amount of possible outcomes are the amount of cards, 52. The probability of selecting a black eight or a black king is:
[tex]P=\frac{4}{52}=\frac{1}{13}[/tex]