Table of contents
- 1. Intro to Stats and Collecting Data24m
- 2. Describing Data with Tables and Graphs1h 55m
- 3. Describing Data Numerically53m
- 4. Probability1h 29m
- 5. Binomial Distribution & Discrete Random Variables1h 16m
- 6. Normal Distribution and Continuous Random Variables58m
- 7. Sampling Distributions & Confidence Intervals: Mean1h 3m
- 8. Sampling Distributions & Confidence Intervals: Proportion1h 5m
- 9. Hypothesis Testing for One Sample1h 1m
- 10. Hypothesis Testing for Two Samples2h 8m
- 11. Correlation48m
4. Probability
Multiplication Rule: Independent Events
Problem 4.2.26b
Textbook Question
Alarm Clock Life Hack Each of us must sometimes wake up early for something really important, such as a final exam, job interview, or an early flight. (Professional golfer Jim Furyk was disqualified from a tournament when his cellphone lost power and he overslept.) Assume that a battery-powered alarm clock has a 0.005 probability of failure, a smartphone alarm clock has a 0.052 probability of failure, and an electric alarm clock has a 0.001 probability of failure.
b. If you use a battery-powered alarm clock and a smartphone alarm clock, what is the probability that they both fail? What is the probability that both of them do not fail?

1
Step 1: Understand the problem. We are tasked with finding two probabilities: (1) the probability that both a battery-powered alarm clock and a smartphone alarm clock fail, and (2) the probability that neither of them fails. The failure probabilities are given as 0.005 for the battery-powered alarm clock and 0.052 for the smartphone alarm clock.
Step 2: To calculate the probability that both fail, use the multiplication rule for independent events. The formula is P(A and B) = P(A) * P(B), where A is the event that the battery-powered alarm clock fails, and B is the event that the smartphone alarm clock fails. Substitute the given probabilities into the formula: P(both fail) = 0.005 * 0.052.
Step 3: To calculate the probability that neither fails, first find the probability that each clock does not fail. For the battery-powered alarm clock, the probability of not failing is 1 - 0.005 = 0.995. For the smartphone alarm clock, the probability of not failing is 1 - 0.052 = 0.948.
Step 4: Use the multiplication rule for independent events again to find the probability that neither fails. The formula is P(neither fails) = P(not A) * P(not B), where not A and not B are the events that the battery-powered alarm clock and the smartphone alarm clock do not fail, respectively. Substitute the probabilities: P(neither fails) = 0.995 * 0.948.
Step 5: Summarize the results. You now have the formulas to calculate both probabilities: (1) P(both fail) = 0.005 * 0.052, and (2) P(neither fails) = 0.995 * 0.948. Perform the calculations to find the final numerical values if needed.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Probability of Independent Events
In probability theory, independent events are those whose outcomes do not affect each other. For example, the failure of a battery-powered alarm clock does not influence the failure of a smartphone alarm clock. To find the probability of both events occurring, you multiply their individual probabilities. This concept is crucial for calculating the likelihood of multiple alarms failing simultaneously.
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Complementary Probability
Complementary probability refers to the likelihood of an event not occurring. If the probability of an event happening is P, then the probability of it not happening is 1 - P. This concept is essential for determining the probability that both alarm clocks do not fail, as it allows us to calculate the complement of their failure probabilities.
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Complementary Events
Joint Probability
Joint probability is the probability of two events occurring at the same time. In this scenario, we are interested in the joint probability of both the battery-powered and smartphone alarm clocks failing. This is calculated by multiplying the individual probabilities of failure for each clock, which provides insight into the overall risk of relying on both alarms.
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