Bayes theorem is based on the concept of conditional probability.

When a question is from Bayes theorem, then in the question some events has already occurred.

Now if this event has occurred, then we have to find the probability of another event.

Example: There are 3 bags, each containing 5 white balls and 3 black balls. Also there are 2 bags, each containing

2 white balls and 4 black balls. A ball is drawn and found it is white. Find the probability that this white ball is from a bag of the first group.

Here when a ball is drawn it is found white. This event is already known.

So we apply Bayes theorem in this example.