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Dependent Events and the Monty Hall Problem

Lesson Plan

Dependent Events and the Monty Hall Problem


This lesson builds on students’ current knowledge of probability. Students will:

  • understand the different definitions involved with compound probability.

  • calculate the different types of probability including intersections and unions.

  • calculate theoretical and experimental probabilities.

Essential Questions

How can data be organized and represented to provide insight into the relationship between quantities?
How can probability and data analysis be used to make predictions?
How does the type of data influence the choice of display?
How precise do measurements and calculations need to be?
In what ways are the mathematical attributes of objects or processes measured, calculated and/or interpreted?
What makes a tool and/or strategy appropriate for a given task?
  • What differentiates an independent event from a dependent event and how are the probabilities of each calculated?


  • Compound Event: An event made up of two or more simple events. [IS.1 - Preparation]

  • Independent Event: Two events in which the outcome of one event does not affect the outcome of the other.

  • Event: An outcome whose probability can be obtained from a single occurrence.

  • Experimental Probability: A statement of probability based on the results of a series of trials.

  • Theoretical Probability: A statement of the probability of an event without doing an experiment or analyzing data. The mathematical probability of an event is represented by a real number, p, such that , where an impossible event is 0 and a certain event is 1.

  • Dependent Event: Two events in which the outcome of one event affects the outcome of the other.

  • Complement: The negation of the occurrence of an event; the complement of event A is event A not occurring.

  • Factorial: The product of all the integers less than or equal to the given integer; for example, the factorial of 5 is 5 × 4 × 3 × 2 × 1 and is expressed as 5!

  • Permutation: An ordered arrangement of all or part of a set of objects.

  • Combination: Any selection of one or more members of a set of objects without regard to order.


90–120 minutes [IS.2 - All Students]

Prerequisite Skills

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Formative Assessment

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    • Think-Pair-Share activity supports students evaluating each other’s work. [IS.14 - Struggling Learners]

    • Teacher observations during group activities and class discussion give the teacher immediate and useful information on student engagement. [IS.15 - Struggling Learners]

    • Exit Ticket activity (M-A2-1-1_Lesson 1 Exit Ticket.doc and M-A2-1-1_Lesson 1 Exit Ticket KEY.doc) gives the teacher a record of student performance. [IS.16 - All Students]

Suggested Instructional Supports

  • View
    Active Engagement, Modeling

    [IS.13 - All Students]


    Students acquire an understanding of the differences between independent and dependent events, theoretical and experimental probabilities, and learn to use the tools to calculate compound and conditional probabilities.


    Game show: Let’s Make a Deal sets the stage to introduce the working operation of a compound probability.


    Vocabulary references (event, independent, dependent, conditional, theoretical, experimental, Fundamental Counting Principle, factorial permutation, combination) from provide language links that relate the language of probability to student experiences


    Group activity: card deck probability questions from random card draws direct student attention to outcomes that can be measured and recorded.


    Lesson 1 Exit Ticket evaluates students’ understanding of the computations that support theoretical probability and gives them a basis for comparing outcomes.


    Group and partner work help students help each other; specific vocabulary words communicate mathematical ideas; and note-taking skills enhance the learning experience and create a useful resource.


    The lesson begins with a game activity. Vocabulary connects important words to concepts. Activities and questions allow students open-ended exploration of probability. Review and evaluation show students what they learned and anticipate the next topics in probability.


    IS.1 - Preparation
    Consider using graphic organizers (e.g., Frayer Model, Verbal Visual Word Association, Concept Circles) to introduce and review key vocabulary prior to the lesson.  
    IS.2 - All Students
    Consider preteaching the concepts critical to this lesson, including the use of hands-on materials. Throughout the lesson (based upon the results of formative assessment), consider the pacing to be flexible to the needs of the students. Also consider the need for reteaching and/or review both during and after the lesson as necessary.  
    IS.3 - Struggling Learners
    Consider providing struggling students with time to preview/review information on dependent events and probability at .
    IS.4 - All Students
    Consider using the “Stick or Switch” activity  (see  found on PA’s SAS) to provide a concrete representation of probability and dependent events.  
    IS.5 - Struggling Learners
    Consider providing a graphic organizer to help struggling students organize and learn the vocabulary associated with probability.  
    IS.6 - All Students
    Consider providing students a copy of this notation and a brief explanation so students can focus on learning instead of copying the information into their notes.  
    IS.7 - Struggling Learners
    Consider using an interactive web site to assist struggling students learn about dependent events and probability. See ,as an example.  
    IS.8 - Struggling Learners
    Peer-mediated instruction (i.e. peer tutoring and cooperative learning groups) can be effective ways for struggling students to enhance their understanding of probability and skills calculating probability of dependent events through practice when peer-mediated instructional activities are well-planned and highly structured  
    IS.9 - All Students
    Instruction that emphasizes the development of calculating probabilities through the use of concrete level instruction positively impacts their development of these skills for the student.  
    IS.10 - All Students
    Consider using a concrete model of the problem and then supporting the students as they work towards the abstract level of probability calculations.  
    IS.11 - Struggling Learners
    Consider providing struggling students with an example of the calculations for the probability of this simulation (I Do, We Do, You Do).  
    IS.12 - All Students
    Consider establishing the groups before instruction takes place so that the teacher has control over who works together.  
    IS.13 - All Students
    Also consider Think-Pair-Share, Random Reporter, Think Alouds, Math Journal, and use of graphic organizers. Information on Think-Pair-Share and  Random Reporter can be found on the SAS website at,  
    IS.14 - Struggling Learners
    Consider facilitating a discussion consisting of advancing / assessing questions and student/teacher Think Alouds  that help struggling students make connections as they deepen their understanding of working with probability. Consider viewing the publication, Teachers’Desk Reference: Essential Practices for Effective Mathematics Instruction in order to review the sections on formative assessment as well as assessing and advancing questions. This publication can be found at:  
    IS.15 - Struggling Learners
    Prior to teaching this lesson, consider the prior knowledge of struggling students as well as misconceptions or preconceptions that are likely to surface. Plan how to correct misconceptions and/or preconceptions and connect current concepts to prior knowledge.  
    IS.16 - All Students

    For additional information on formative assessment, please visit and click on the “Fair Assessments” tab

Instructional Procedures

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    [IS.3 - Struggling Learners] After this lesson, students will understand the difference between independent and dependent events and theoretical and experimental probability. They will be able to calculate compound probabilities as well as conditional probabilities. Students learn best when they can see a purpose for learning the content. Probability happens all around us, and if the examples are situations that students can relate to, their motivation to learn increases as well as their success in the unit concepts.

    Introduce the game show “Let’s Make a Deal” to the class. [IS.4 - All Students]

    Suppose you are on a game show, and you are given the choice of three doors: behind one door is a car; behind the others, goats. You pick a door, say No. 1, and the host, who knows what is behind the doors, opens another door, say No. 3, that has a goat. He then says to you, ‘Do you want to pick door No. 2?’Is it to your advantage to switch your choice?”

    Go through a few demonstrations of the game and have students be the contestants. Use cardboard boxes with a prize in one of the boxes and two stuffed-animal goats (or pictures of a goat) in the other two. Make sure to mix up what is in each box after each contestant chooses. Tell students who are watching to jot down notes and brainstorm the probability of winning the prize.

    After the demonstration, have a discussion about the probability of winning. Show the graphic that breaks down the individual outcomes with and without switching.

    After choosing a box, the host opens another one and gives you the choice to switch boxes. What is the probability that you will win the prize?” The common misconception behind this problem is that the probability is 1/2 and most likely students will say 1/2. The incorrect assumption is that there are two remaining doors when there are actually three and the probability is 1/3.

    Show the Monty Hall Problem video on YouTube:

    Discuss their reactions to the problem and the video. They can also see a demonstration at

    Students should put the following vocabulary into their notes. [IS.5 - Struggling Learners] The vocabulary is from

    An event is the outcome of an experiment. For example, flipping a coin results in two events: heads or tails. [IS.6 - All Students]

    P(A) is the notation for the probability of event A occurring.

    1 − P(A) is the notation for the probability of event A not occurring.

    P(A ∩ B) is the notation for the probability of event A and event B occurring.

    P(A ∩ B) = P(A) × P(B)

    P(A U B) is the notation for the probability of event A or event B occurring.

    P(A U B) = P(A) + P(B) − P(A ∩ B)

    Two events are independent if one event occurs and it does not affect the probability of the other event. [IS.7 - Struggling Learners] For example, if you flip a coin twice, getting a heads on the first flip does not affect the probability of getting a tails on the second flip. It’s still 1/2.

    This means that if two events are dependent, then the occurrence of one event affects the probability of the other event. For example, the probability of you picking a red card from a deck of 52 cards is 26/52 = 0.5 (since half the cards in the deck are red). If you keep that card out, the probability of you picking another red card is now 25/51 = 0.49 because the card you’ve taken out is one of the red cards (26 – 1) and one of the total 52 cards (52 – 1).

    The Monty Hall problem is an example of conditional probability. Conditional probability is the probability of one event happening given another event has already occurred.

    P(A|B) is the notation for the probability of event A occurring given event B has already occurred: P(A|B) = P(A ∩ B) ÷ P(B).

    Theoretical probability is the predicted outcome, whereas the experimental probability is the actual outcome. For example, the theoretical probability of rolling a 4 on a number cube is 1/6 = 0.167 (since the 4 appears on one of the six sides). If you performed an experiment and you rolled a 4 once out of ten rolls or 1/10 = 0.10, this is the experimental probability.

    The Fundamental Counting Principle is a method used to calculate all the possible combinations of a given event. For example, if you are getting ready for school and you have three different pants to wear and five different shirts to wear, you have 3 × 5 or 15 different outfits.

    The product of an integer and all the integers less than it and greater than zero is called a factorial. It is denoted “n!.” For example, 4! = 4 × 3 × 2 × 1 = 24. Notice that there is one way to arrange zero objects and that expression as a factorial is 0! = 1.

    A permutation is the number of ways things can be arranged in order. For example, if Alex, Ben, and Colleen are running for student council president, vice-president, and treasurer, there are 6 ways to arrange them in the student council: ABC, ACB, BAC, BCA, CAB, and CBA. Think of the p in permutation as representing position.

    A combination is the number of ways things can be arranged where order does not matter. For example, if Alex, Ben, and Colleen are trying out for 2 spots on a sports team, there are only 3 ways they can make the team: AB, AC or BC. Alex and Ben making the team is the same as Ben and Alex making the team (order does not matter). Think of the c in combination as representing committee.

    Activity 1: Pairs

    If possible, set students up in a computer lab or have students use laptops. If this is not possible, then you can do this activity as a whole class. [IS.8 - Struggling Learners] Hand out the Spinner Activity Worksheet (M-A2-1-1_Spinner Activity Worksheet.doc).

    Go to It has a spinner on it that can be adjusted for the number of sectors. Instead of using the Web site’s numbers, have students compute the theoretical and experimental probability.

    Activity 2: The Birthday Problem

    Ask students to think about the probability of any two students in class having their birthdays on the same day of the week, (i.e., Monday, Tuesday, etc.).

    The probability that two students have a birthday on the same day of the week is (7/7) × (1/7) = .14285 . You start with 7/7 because the first student can have his/her birthday on any day of the week, and you multiply by 1/7 because the second student must have his/her birthday on the same day of the week. To get the total probability, the probabilities of both events are multiplied together. Sometimes working with the probability of an event not happening is easier to calculate than the probability of an event happening. For example, the probability that two people do not have the same birthday is (365/365) × (364/365) = 0.99726. The probability of the first student’s birthday is 365/365, as the student’s birthday can be any day of the year. The probability of the second student’s birthday is 364/365, as it can be any day except one. Therefore, the probability of two people having the same birthday is 1 − 0.99726 = 0.00274.”

    Go to and show students the demonstration. [IS.9 - All Students] Change the number of people and also use the number of students in the classroom. This gives them the theoretical probability. Then have students calculate the experimental probability.

    Activity 3: Think-Pair-Share

    Discuss sample size with students. In the following problem, the sample size starts with 20 candies. Each time a candy is taken, the sample size is reduced by one unless it is replaced. The concept of replacement is important and should be emphasized with reference to an experiment. A bag contains one blue and one red marble. Randomly drawing the blue marble has a probability of 0.5. Not replacing the blue marble for the next random draw means that the probability of drawing the red marble is 1, and the probability of drawing the blue marble is 0. However, if you replace the blue marble after the first draw, the probability is the same as for the original draw, 0.5. Point out that these types of activities are different from coin flips or rolls of one or more number cubes because replacement does not apply to them. Also point out the difference between a flip and a roll. One coin flip has 2 outcomes; one roll has 6 outcomes.

    Present to students the following problem: “You are sitting in front of the principal’s desk when you notice she has a jar of candy. [IS.10 - All Students] You count 10 strawberry-flavored candies, 5 vanilla-flavored, 4 blueberry-flavored, and 1 chocolate-flavored in the jar.”

    1. What is the probability of each flavor? [IS.11 - Struggling Learners]

    P(S) = 10/20      P(V) = 5/20

    P(B) = 4/20     P(C) = 1/20

    2. The principal tells you that you may randomly pick only one candy. What is the probability of picking a blueberry or a strawberry? P(B U S) =

    P(B) + P(S) = ( 4/20 + 10/20 ) = 14/20

    3a. She tells you that you may randomly pick two candies. After picking the first candy, it is placed back in the jar. What is the probability of picking one vanilla and one chocolate?

    P(V ∩ C) =( 5/20 × 1/20 ) = 5/400 = 1/80 = 0.0125

    3b. What is the probability of picking one vanilla and one chocolate if the first candy is not placed back in the jar? (Remember that the sample size, the denominator, is reduced by one for the second candy. The first product is the probability of picking a vanilla first and then a chocolate. The second product is the probability of pulling a chocolate first and then a vanilla.)


    ( 5/20 × 1/19 ) + ( 1/20 × 5/19 ) = 5/380 + 5/380 = 10/380 = 0.02632

    4a. What is the probability of randomly picking one blueberry given that you have already picked a vanilla candy and replaced it in the jar?

    P(B|V) = ( 4 blueberry/20 candies ) = (0.2)

    4b. What is the probability of randomly picking one blueberry given that you have already picked a vanilla candy and have not replaced it?

    P(B|V) = ( 4 blueberry/19 candies ) =  (4/19).

    5. Let’s say that each candy had a number on it in the jar. The ten strawberry candies were numbered 1 through 10, the five vanilla candies were numbered 1 through 5, the four blueberry candies were numbered 1 through 4 and the chocolate was numbered 1. Using the fundamental counting principle, how many ways can you pick one of each color? (200)

    Activity 4: Groups

    Divide the class into groups of four. [IS.12 - All Students] Give them each a deck of cards. Allow them 10 minutes to write ten probability questions regarding drawing from the deck. For example, what is the probability of picking one red card? What is the probability of picking a red card and a four? After they have written their questions, have each group share one with the whole class and write them on the board. When ten questions are on the board, the groups have to solve the ten questions. Review and discuss the answers as a whole class. Collect each group’s questions and use them as a warm-up the next day or on a quiz.

    An Exit Ticket (M-A2-1-1_Lesson 1 Exit Ticket.doc and M-A2-1-1_Lesson 1 Exit Ticket KEY.doc) is a quick way to evaluate whether students understand the concepts.


    Use the following strategy to tailor the lesson to meet the needs of your students throughout the year.

    • Routine: Group and partner work is used throughout so that students can help each other. Emphasis should be placed on communicating mathematical ideas with the specific vocabulary words appropriate to the concepts. The lesson requires accurate note-taking skills to enhance the learning experience while creating a useful resource (notes).

    This lesson is a building block for the lessons to come and is necessary for students to understand probability and how it is used in real-world situations. It begins with a game show activity that engages students and piques their interests. The teacher then introduces some vocabulary that students will need to know before the activities. The activities are simple and provide students time to explore probability. Students are given time to review the lesson’s concepts and are given timely feedback. The teacher also explains how students will use this information in the next lesson.

Related Instructional Videos

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DRAFT 05/28/2010
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