Lesson 2: 5.2 Basic Probability Rules
Duration of Days: 3
Lesson Objective
Students will be able to apply the fundamental rules of probability, including the range of possible values, the sum of probabilities in a sample space, and the complement rule.
1. Why is it mathematically impossible for a weather forecaster to say there is a 110% chance of rain?
2. If the probability of an event is very close to 0, does that mean it will never happen?
Sample space
Probability model
Event
Complement
Mutually exclusive
HSS-CP.A.1: Describe events as subsets of a sample space using characteristics of the outcomes.
The SAT frequently tests the basic definition of probability (Desired/Total). Understanding the complement rule is a major time-saver on the SAT; often, it’s easier to calculate the probability of what you don’t want and subtract it from 1.
This section establishes the "legal limits" of probability. It ensures students understand that all probabilities must live between 0 and 1 and that the entire sample space must account for 100% of possibilities.
The Problem: A bag contains 10 red marbles, 20 blue marbles, and 15 green marbles.
Task: Create a probability model for choosing one marble. Then, find the probability that you do not choose a red marble.
Negative Probabilities
"Disjoint" vs. "Independent"
Support: Use a physical "Probability Number Line" on the floor. Have students stand on the line based on phrases like "Unlikely," "50-50," or "Highly Probable."
Extension: Challenge students to create a sample space for a complex event, like flipping a coin four times, and prove that the sum of all outcomes is 1.
Teacher assigns examples from the textbook and other resources.
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