A Youtube video from statisticsfun, Tutorial on how to calculate conditional probability (Bayes Theorem) for two events P(A), P(B), P(B|A) with two examples

Conditional Probability When dealing with two events (usually called A and B), sometimes the events are so related to each other, that the probability of one depends on whether the other event has occurred. When we talk about probabilities based on the fact that something else has already happened we call this conditional probability.

A tutorial with interactive exercises from algebralab.org