Understanding probability of occurring of an event is an important part to understand in Machine learning and Data Science.

Taking a simple example, say we need to figure out probability of two dice getting rolled and sum is greater or equal to 10, i.e. sum can be 10, 11, 12. This can occur in following manner

{{4,6},{5,5},{5,6},{6,4},{6,5},{6,6}} =6

so we have 6 outcomes which can fulfill this condition.

Total outcomes

{{1,1},{1,2},…,{5,5},{5,6}} = 36

So probability of sum to be more than or equal to 10 is

desirable outcomes/ total outcomes = 6/36 = 1/6

After the simple probability, comes **conditional probability**. There might be some preconditions for an event to occur, which increases probability of the event to occur. For example, probability of “it will rain” gets increased if we already have an event “it will be cloudy”.

Going back to previous example, lets say we add a condition “figure out probability of two dice getting rolled and sum is greater or equal to 10 – when the first dice already has 5”

P(A)= P(A and B)/P(B)

A = Event we need to check probability for, in this case, probability of sum greater than 10 when first dice has 5

B = Event that First dice has five

A and B = probability of both events occurring together.

We already know total outcomes for two dice being rolled is 36.

Probability for Event B, i.e. first dice has 5, we have {{5,1},{5,2},{5,3},{5,4},{5,5},{5,6}}

i.e. 6/36 or 1/6

Probability for A and B, i.e. first dice has 5 and sum is greater or equal to 10 {{5,5},{5,6}}

i.e. 2/36 or 1/18

putting these values in formula

P(A)= P(A and B)/P(B)

P(A)= (1/18)/(1/6) or (6/18) or (1/3)

Finally coming to **Bayes Theorem**, this gives relation between conditional probability and its reverse.

P(A/B) = (P(B/A)*(PA))/P(B)

or

P(A/B)*P(B) = P(B/A)*(PA)

A good example is here

But I will stick to simpler one

You have 2 bags, say X and Y. X has 6 Green and 6 Red balls. Y has 4 Red and 8 Green Balls. one of the bags is randomly chosen and a ball is picked randomly. If the ball is Red, what is probability that it was taken from bag X.

We need to find P(Bag X/ Ball is Red)

As per Bayes we have

P(Bag X/ Ball is Red) = (P(Ball is Red/Bag X)*P(Bag X))/ P(Ball is Red)

As bag is chosen randomly, we know

P(Bag X) and P(Bag y) both is 1/2

P(Ball is Red/Bag X) = Bag X has 12 balls of which 6 are Red, so 6/12 = 1/2

P(Ball is Red) = Red ball is chosen from X bag OR Red ball is chosen from Y bag

=P(Ball is Red/Bag X)*P(Bag X) + P(Ball is Red/Bag Y)*P(Bag Y)

= 6/12*1/2 + 4/12*1/2

= 10/24 = 5/12

P(Bag X/ Ball is Red) = (P(Ball is Red/Bag X)*P(Bag X))/ P(Ball is Red)

= (1/2*1/2)/5/12 = (1/4)/(5/12) = 3/5