Reasoning
Here's the reasoning behind why it's usually easier to figure out the probability of not hitting:
We want to know how often at least one A or K hits. There are a number of different combinations of Aces and Kings. There could be 0 Aces and 0 Kings, 1 Ace and 0 Kings, 0 Aces and 1 King, 2 Aces and 0 Kings, etc etc etc. In all there are 9 different combinations of Aces and Kings, but only 1 way to have 0 Aces AND 0 Kings.
In order to figure out the probability of the board showing at least one Ace or King, we could calculate all of the probabilities for all boards that have at least one A or K and add them all together. But this means 9 probabilities to calculate.
To make it easier, we just figure out the probability of 0 Aces and 0 Kings. If the board doesn't have 0 Aces AND 0 Kings, then it has at least one of them. So we subtract our number from 1 (because adding all probabilities together equals 1) to get our answer.
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