CK-12-Basic Probability and Statistics Concepts - A Full Course

2.7. Conditional Probability http://www.ck12.org

Guided Practice

At Bluenose High School, 90% of the students take Physics and 35% of the students take both Physics and Statistics.
What is the probability that a student from Bluenose High School who is taking Physics is also taking Statistics?

Answer:

Step 1:List what you know.

P(physics) = 0. 90

P(physics and statistics) = 0. 35

Step 2:Calculate the probability of choosing Statistics as a second course when Physics is chosen as a first course.

P(statistics|physics) =

P(physics and statistics)

P(physics)

P(statistics|physics) =

0. 35

0. 90

P(statistics|physics) = 0. 388

P(statistics|physics) =39%

Step 3:Write your conclusion.

Therefore, the probability that a student from Bluenose High School who is taking Physics is also taking Statistics
is 39%.

Practice

1. 2 fair dice are rolled. What is the probability that the sum is even given that the first die that is rolled is a 2?


2. 2 fair dice are rolled. What is the probability that the sum is even given that the first die rolled is a 5?


3. 2 fair dice are rolled. What is the probability that the sum is odd given that the first die rolled is a 5?


4. Steve and Scott are playing a game of cards with a standard deck of playing cards. Steve deals Scott a black
   king. What is the probability that Scott’s second card will be a red card?


5. Sandra and Karen are playing a game of cards with a standard deck of playing cards. Sandra deals Karen a
   red seven. What is the probability that Karen’s second card will be a black card?