http://www.ck12.org Chapter 2. Conditional Probability
Now let’s add up all the probabilities.
22 .68%+ 31 .32%+ 22 .08%+ 23 .92%=100%
All of the probabilities do, in fact, add up to 100%.
Practice
- A bag contains 3 red balls and 4 blue balls. Thomas reaches in the bag and picks a ball out at random from
the bag. He places it back into the bag. Thomas then reaches in the bag and picks another ball at random.
(a) Draw a tree diagram to represent this problem.
(b) What is the probability that Thomas picks:
i. 2 red balls
ii. a red ball in his second draw - A teacher has a prize box on her front desk for when students do exceptional work in math class. Inside the box
there are 20 math pencils and 10 very cool erasers. Janet completed a challenge problem for Ms. Cameron,
and Ms. Cameron rewarded Janet’s innovative problem-solving approach with a trip to the prize box. Janet
reaches into the box and picks out a prize and then drops it back in. Then she reaches in again and picks out a
prize a second time.
a. Draw a tree diagram to represent this problem.
b. What is the probability that Janet reaches into the box and picks out an eraser on the second pick? - Students in BDF High School were asked about their preference regarding the new school colors. They were
given a choice between green and blue as the primary color and red and yellow as the secondary color. The
results of the survey are shown in the tree diagram below. You can see that 75% of the students choose green
as the primary color. Of this 75%, 45% chose yellow as the secondary color. What is the probability that a
student in BDF High School selected red as the secondary color if he or she chose blue as the primary color? - In question 3, what color combination do the students in BDF High School want the most?
- According to the following tree diagram, what is the probability of getting 1 head and 1 tail when flipping a
coin 2 times? Note that 1 head and 1 tail can mean a head and then a tail or a tail and then a head.