Lesson 9: Multi-step Experiments

Let’s look at probabilities of experiments that have multiple steps.

9.1: True or False?

Is each equation true or false? Explain your reasoning.

$8=(8+8+8+8)\div3$

$(10+10+10+10+10)\div5=10$

$(6+4+6+4+6+4)\div6=5$

9.2: Spinning a Color and Number

The other day, you wrote the sample space for spinning each of these spinners once.

What is the probability of getting:

  1. Green and 3?
  2. Blue and any odd number?
  3. Any color other than red and any number other than 2?

Two different circular spinners. The spinner on the left is divided into four equal parts. The first part is red and labeled "R," the second part is blue and labeled “B,” the third part is green and labeled “G,” and the fourth part is white and labeled "W.” The pointer is in the part labeled “W.” The spinner on the right is divided into five equal parts. Starting from the top right, and moving clockwise, the first part is labeled 1, the second, 2, the third, 3, the fourth, 4, and the fifth, 5. The pointer is in the part labeled “5.”

 

9.3: Cubes and Coins

The other day you looked at a list, a table, and a tree that showed the sample space for rolling a number cube and flipping a coin.

  1. Your teacher will assign you one of these three structures to use to answer these questions. Be prepared to explain your reasoning.

    1. What is the probability of getting tails and a 6?

    2. What is the probability of getting heads and an odd number?

      Pause here so your teacher can review your work.

  2. Suppose you roll two number cubes. What is the probability of getting:

    1. Both cubes showing the same number?
    2. Exactly one cube showing an even number?
    3. At least one cube showing an even number?
    4. Two values that have a sum of 8?
    5. Two values that have a sum of 13?
  3. Jada flips three quarters. What is the probability that all three will land showing the same side?

9.4: Pick a Card

Imagine there are 5 cards. They are colored red, yellow, green, white, and black. You mix up the cards and select one of them without looking. Then, without putting that card back, you mix up the remaining cards and select another one.

  1. Write the sample space and tell how many possible outcomes there are.
  2. What structure did you use to write all of the outcomes (list, table, tree, something else)? Explain why you chose that structure.

  3. What is the probability that:

    1. You get a white card and a red card (in either order)?

    2. You get a black card (either time)?

    3. You do not get a black card (either time)?

    4. You get a blue card?

    5. You get 2 cards of the same color?

    6. You get 2 cards of different colors?

Summary

Suppose we have two bags. One contains 1 star block and 4 moon blocks. The other contains 3 star blocks and 1 moon block.

If we select one block at random from each, what is the probability that we will get two star blocks or two moon blocks?

Two bags of blocks. The bag on the left contains 5 blocks: 1 star block and 4 moon blocks. The bag on the right contains 4 blocks: 3 star blocks and 1 moon block.

To answer this question, we can draw a tree diagram to see all of the possible outcomes.

A tree diagram. The first choice has 5 branches, representing the 5 blocks in the bag: one branch is labeled “star,” the other 4 are labeled “moon.” Each of these branches has 4 branches, representing the 4 blocks in the second bag. 3 branches are labeled “star” and one is labeled “moon.” The word “star” in the first choice, and the 3 “star” choices branching from it are highlighted gold. From the first choice, the word “moon” is highlighted blue on each of the four remaining branches. From each of those branches, the one choice of “moon” for each is also highlighted blue.

There are $5 \boldcdot 4 = 20$ possible outcomes. Of these, 3 of them are both stars, and 4 are both moons. So the probability of getting 2 star blocks or 2 moon blocks is $\frac{7}{20}$.

In general, if all outcomes in an experiment are equally likely, then the probability of an event is the fraction of outcomes in the sample space for which the event occurs.

Practice Problems ▶