The page on 429-5 said:
P(A|B) = Number of successful outcomes / Total number of possible outcomes
Then: P(A|B) = number of successful outcomes / card(B)
Then: total number of possible outcomes = card(A∩B)
Then you arrived at following: P(A|B) = card(A∩B) / card(B)
How is this? Going by the steps, it should be P(A|B) = card(B) / card(A∩B)
I think your final formula is correct, but the steps seem confusing to me.
In the end, I am able to get the answer myself, but the formula derivation part is confusing.
Thanks