Illustrative Mathematics Grade 7, Unit 8, Lesson 4: Estimating Probabilities Through Repeated Experiments

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Illustrative Math
Grade 7

Lesson 4: Estimating Probabilities Through Repeated Experiments

Let’s do some experimenting.

Illustrative Math Unit 7.8, Lesson 4 (printable worksheets)

Lesson 4 Summary

A probability for an event represents the proportion of the time we expect that event to occur in the long run. For example, the probability of a coin landing heads up after a flip is , which means that if we flip a coin many times, we expect that it will land heads up about half of the time.

Even though the probability tells us what we should expect if we flip a coin many times, that doesn’t mean we are more likely to get heads if we just got three tails in a row. The chances of getting heads are the same every time we flip the coin, no matter what the outcome was for past flips.

Lesson 4.1 Decimals on the Number Line

1. Locate and label these numbers on the number line.
   a. 0.5
   b. 0.75
   c. 0.33
   d. 0.67
   e. 0.25
2. Choose one of the numbers from the previous question. Describe a game in which that number represents your probability of winning.

• See Video for Whole Lesson

Lesson 4.2 In the Long Run

Mai is playing a game where she will win only if she rolls a 1 or a 2 with a standard number cube.

1. List the outcomes in the sample space for rolling the number cube.
2. What is the probability Mai will win the game? Explain or show your reasoning.
3. If Mai is given the option to flip a coin and win if it comes up heads, is that a better option for her to win?
   This applet displays a random number from 1 to 6, like a number cube. Mai won with the numbers 1 and 2, but you can choose any two numbers from 1 to 6. Record them in the boxes in the center of the applet.
   
4. Click the Roll button for 10 rolls, and then answer the questions below.
   Open Applet
5. If the roll stops on one of your winning numbers, what happens in the table?
6. What appears to be happening with the points on the graph?
7. a. After 10 rolls, what fraction of the total rolls were a win?
   b. How close is this fraction to the probability that Mai will win?
8. Roll the number cube 10 more times to fill in the table and graph the results, for a total of 20 points on the graph.
9. a. After 20 rolls, what fraction of the total rolls were a win?
   b. How close is this fraction to the probability that Mai will win?

Lesson 4.3 Due For a Win

1. For each situation, do you think the result is surprising or not? Is it possible? Be prepared to explain your reasoning.
   a. You flip the coin once, and it lands heads up.
   b. You flip the coin twice, and it lands heads up both times.
   c. You flip the coin 100 times, and it lands heads up all 100 times.
2. If you flip the coin 100 times, how many times would you expect the coin to land heads up? Explain your reasoning.
3. If you flip the coin 100 times, what are some other results that would not be surprising?
4. You’ve flipped the coin 3 times, and it has come up heads once. The cumulative fraction of heads is currently 1/3. If you flip the coin one more time, will it land heads up to make the cumulative fraction 2/4?

Lesson 4 Practice Problems

1. A carnival game has 160 rubber ducks floating in a pool. The person playing the game takes out one duck and looks at it.

• If there’s a red mark on the bottom of the duck, the person wins a small prize.
• If there’s a blue mark on the bottom of the duck, the person wins a large prize.
• Many ducks do not have a mark.
  After 50 people have played the game, only 3 of them have won a small prize, and none of them have won a large prize.
  Estimate the number of the 160 ducks that you think have red marks on the bottom. Then estimate the number of ducks you think have blue marks. Explain your reasoning.

2. Lin wants to know if flipping a quarter really does have a probability of 1/2 of landing heads up, so she flips a quarter 10 times. It lands heads up 3 times and tails up 7 times. Has she proven that the probability is not 1/2? Explain your reasoning.
3. A spinner is spun 40 times for a game. Here is a graph showing the fraction of games that are wins under some conditions.
   Estimate the probability of a spin winning this game based on the graph.
4. Which event is more likely: rolling a standard number cube and getting an even number, or flipping a coin and having it land heads up?5. Noah will select a letter at random from the word “FLUTE.” Lin will select a letter at random from the word “CLARINET.”
   Which person is more likely to pick the letter “E?” Explain your reasoning.

The Open Up Resources math curriculum is free to download from the Open Up Resources website and is also available from Illustrative Mathematics.

Try the free Mathway calculator and problem solver below to practice various math topics. Try the given examples, or type in your own problem and check your answer with the step-by-step explanations.

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