Joint Probability
Joint probability is the probability of two events happening together.
The two events are usually designated event A and event B. In probability terminology, it can be written as:
Anyway, if an event A and B are idenpendent then
, and therefore,
become
Example: The probability that a card is a five and black = p(five and black)
Here p(five|black) is the conditional probability of drawing a five given that the card is black, and p(black) is the probability of drawing a black card.
The probability of drawing a black card from a standard deck of 52 playing cards is:
p(black) = Number of black cards / Total number of cards
= 26 / 52
= 1/2
The probability of drawing a five given that the card is black is:
p(five|black) = Number of black fives / Number of black cards
= 2 / 26
= 1/13
Therefore, the joint probability of drawing a five and a black card is:
p(five and black) = p(five|black) * p(black)
= (1/13) * (1/2)
= 1/26
Another way
2/52 = 1/26
(There are two black fives in a deck of 52 cards, the five of spades and the five of clubs).