429-5 explanation seems confusing to me

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

Hi @kunalsoni:

Please attach the mission link as per these guidelines.

Here’s the link: