Learn on PengiIllustrative Mathematics, Grade 7Chapter 8: Probability and Sampling

Lesson 2: Probabilities of Multi-step Events

In this Grade 7 lesson from Illustrative Mathematics Chapter 8, students learn how to identify and organize sample spaces for multi-step experiments using three methods: lists, tables, and tree diagrams. The lesson applies these strategies to real-world scenarios like choosing meals or clothing combinations, and introduces the multiplication principle to count total outcomes. Students then use sample spaces to calculate probabilities of specific outcomes in multi-step events.

Section 1

Creating Tree Diagrams for Compound Events

Property

A tree diagram systematically displays all possible outcomes of compound events by creating branches for each outcome of the first event, then extending branches for each outcome of subsequent events from every existing branch.

Examples

Section 2

Probabilities of compound events

Property

Find probabilities of compound events using organized lists, tables, tree diagrams, and simulation. Understand that, just as with simple events, the probability of a compound event is the fraction of outcomes in the sample space for which the compound event occurs.
A compound event is an event that can be viewed as two or more simpler events happening. If the simple events do not influence each other, they are called independent events. For independent events, the probability of the compound event is the product of the probabilities of the simple events.

Examples

  • The probability of rolling a 4 on a die (16\frac{1}{6}) and flipping heads on a coin (12\frac{1}{2}) is 16×12=112\frac{1}{6} \times \frac{1}{2} = \frac{1}{12}.
  • A bag has 3 red and 2 blue marbles. The probability of drawing a red marble, replacing it, and then drawing a blue marble is 35×25=625\frac{3}{5} \times \frac{2}{5} = \frac{6}{25}.
  • A weather forecast gives a 20% chance of rain on Saturday and a 50% chance on Sunday. The probability of rain on both days is 0.20×0.50=0.100.20 \times 0.50 = 0.10, or 10%.

Explanation

A compound event combines two or more simple events. If they are independent (one does not affect the other), find the total probability by multiplying their individual probabilities. It is like a chain reaction of chances!

Section 3

Using Area Models for Probability

Property

An area model, or table, can represent the sample space of a two-step experiment. The rows show the outcomes of the first event, and the columns show the outcomes of the second. Each cell represents one possible outcome in the sample space, and the total number of cells gives the total number of outcomes.

P(event)=Number of favorable outcomesTotal number of outcomes (cells)P(\text{event}) = \frac{\text{Number of favorable outcomes}}{\text{Total number of outcomes (cells)}}

Examples

  • To find the probability of rolling an even number on a die and flipping heads on a coin, we can create a table. The sample space has 6×2=126 \times 2 = 12 total outcomes. The favorable outcomes (Even, H) are (2,H), (4,H), and (6,H). The probability is 312=14\frac{3}{12} = \frac{1}{4}.
  • Two spinners are spun. Spinner A has colors Red and Blue. Spinner B has numbers 1, 2, and 3. The probability of landing on Blue and an odd number (1 or 3) can be found using a 2×32 \times 3 table. There are 2 favorable outcomes (Blue, 1) and (Blue, 3) out of 6 total outcomes. The probability is 26=13\frac{2}{6} = \frac{1}{3}.

Explanation

An area model is a grid used to visualize all possible outcomes of a multi-step experiment. It is especially useful for two-step events, like rolling two dice or flipping a coin twice. By counting the cells that match the desired event and dividing by the total number of cells, you can easily calculate the theoretical probability. This method provides a clear and organized alternative to a tree diagram or simple list.

Book overview

Jump across lessons in the current chapter without opening the full course modal.

Continue this chapter

Chapter 8: Probability and Sampling

  1. Lesson 1

    Lesson 1: Probabilities of Single Step Events

  2. Lesson 2Current

    Lesson 2: Probabilities of Multi-step Events

  3. Lesson 3

    Lesson 3: Sampling

  4. Lesson 4

    Lesson 4: Using Samples

Lesson overview

Expand to review the lesson summary and core properties.

Expand

Section 1

Creating Tree Diagrams for Compound Events

Property

A tree diagram systematically displays all possible outcomes of compound events by creating branches for each outcome of the first event, then extending branches for each outcome of subsequent events from every existing branch.

Examples

Section 2

Probabilities of compound events

Property

Find probabilities of compound events using organized lists, tables, tree diagrams, and simulation. Understand that, just as with simple events, the probability of a compound event is the fraction of outcomes in the sample space for which the compound event occurs.
A compound event is an event that can be viewed as two or more simpler events happening. If the simple events do not influence each other, they are called independent events. For independent events, the probability of the compound event is the product of the probabilities of the simple events.

Examples

  • The probability of rolling a 4 on a die (16\frac{1}{6}) and flipping heads on a coin (12\frac{1}{2}) is 16×12=112\frac{1}{6} \times \frac{1}{2} = \frac{1}{12}.
  • A bag has 3 red and 2 blue marbles. The probability of drawing a red marble, replacing it, and then drawing a blue marble is 35×25=625\frac{3}{5} \times \frac{2}{5} = \frac{6}{25}.
  • A weather forecast gives a 20% chance of rain on Saturday and a 50% chance on Sunday. The probability of rain on both days is 0.20×0.50=0.100.20 \times 0.50 = 0.10, or 10%.

Explanation

A compound event combines two or more simple events. If they are independent (one does not affect the other), find the total probability by multiplying their individual probabilities. It is like a chain reaction of chances!

Section 3

Using Area Models for Probability

Property

An area model, or table, can represent the sample space of a two-step experiment. The rows show the outcomes of the first event, and the columns show the outcomes of the second. Each cell represents one possible outcome in the sample space, and the total number of cells gives the total number of outcomes.

P(event)=Number of favorable outcomesTotal number of outcomes (cells)P(\text{event}) = \frac{\text{Number of favorable outcomes}}{\text{Total number of outcomes (cells)}}

Examples

  • To find the probability of rolling an even number on a die and flipping heads on a coin, we can create a table. The sample space has 6×2=126 \times 2 = 12 total outcomes. The favorable outcomes (Even, H) are (2,H), (4,H), and (6,H). The probability is 312=14\frac{3}{12} = \frac{1}{4}.
  • Two spinners are spun. Spinner A has colors Red and Blue. Spinner B has numbers 1, 2, and 3. The probability of landing on Blue and an odd number (1 or 3) can be found using a 2×32 \times 3 table. There are 2 favorable outcomes (Blue, 1) and (Blue, 3) out of 6 total outcomes. The probability is 26=13\frac{2}{6} = \frac{1}{3}.

Explanation

An area model is a grid used to visualize all possible outcomes of a multi-step experiment. It is especially useful for two-step events, like rolling two dice or flipping a coin twice. By counting the cells that match the desired event and dividing by the total number of cells, you can easily calculate the theoretical probability. This method provides a clear and organized alternative to a tree diagram or simple list.

Book overview

Jump across lessons in the current chapter without opening the full course modal.

Continue this chapter

Chapter 8: Probability and Sampling

  1. Lesson 1

    Lesson 1: Probabilities of Single Step Events

  2. Lesson 2Current

    Lesson 2: Probabilities of Multi-step Events

  3. Lesson 3

    Lesson 3: Sampling

  4. Lesson 4

    Lesson 4: Using Samples