Conditional Probability means that we must have two events - call them event A and event B (or whatever two random variables exist in your problem). Given that one event has already happened, or given that one random variable is present, what is the probability that the other will also happen? Or what is the probability that the other random variable will also exist?
Your AP exam formula sheet gives you the conditional probability theorem like this:
This is read as, "The probability of A, given that B has already happened." This is equal to the probability of A and B (numerator) over the probability of B (denominator), where B is the event that already happened.
We usually use a 2-way contingency table when finding conditional probabilities. Find the marginal totals (row and column totals) first. Then, you will use these totals along with the information within the table to find your conditional probabilities.
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