CONDITIONAL PROBABILITY

The probability of an event occurring given that another event has already occurred is called a conditional probability.

Recall that when two events, A and B, are dependent, the probability of both occurring is

P( A and B) = P( A) × P( B given A)

or P( A and B) = P( A) × P( B | A)

If we divide both sides of the equation by P(A) we get

Example:

Susan took two tests. The probability of her passing both tests is 0.6. The probability of her passing the first test is 0.8. What is the probability of her passing the second test given that she has passed the first test?

Solution:

Example:

A bag contains red and blue marbles. Two marbles are drawn without replacement. The probability of selecting a red marble and then a blue marble is 0.2. The probability of selecting a red marble on the first draw is 0.5. What is the probability of selecting a blue marble on the second draw, given that the first marble drawn was red?

Solution:

Example:

What is the probability that the total of two dice will be greater than 9, given that the first die is a 5?

Solution:

Let A = first die is 5

Let B = total of two dice is greater than 9

P(A) =

Possible outcomes for A and B: (5, 5), (5, 6)

P(A and B) =

The following video gives an introduction to conditional probability.

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