Missing Probability Answer Key
About This Worksheet
This worksheet helps twelfth-grade Pre-Calculus students practice finding a missing probability in a discrete probability distribution. Students work through ten tables where one probability is represented by p. Their job is to use the fact that all probabilities in a valid distribution must add up to one.
The problems include decimals and fractions, so students get practice with different number forms. The goal is to make the total probability complete without changing any of the values that are already given.
What Students Practice
Students practice adding the known probabilities first and then subtracting that total from one. The amount left over is the missing probability.
The worksheet also reinforces the idea that a missing probability must still be a valid probability. After finding p, students should make sure it falls between zero and one.
Why This Skill Matters
Missing-probability problems help students understand the structure of a distribution. The table is not just a list of unrelated numbers. All of the probabilities work together to represent the full set of possible outcomes.
This skill also prepares students for more advanced probability tables where they may need to fill in unknown values before calculating expected value or spread.
Common Challenges
Students may add the probabilities incorrectly, especially when fractions and decimals are mixed. Encourage them to use one form consistently when possible.
Another common mistake is setting the known probabilities equal to one without including p. Remind students that the entire distribution, including the missing value, must total one.
How To Use It
Teachers can use this worksheet immediately after reviewing the rules for a valid probability distribution. It works well for guided practice, independent work, homework, or a quick skills check.
Parents and homeschool teachers can say, “Add what you already know, then find what is missing to reach one.” That is the whole idea behind each problem.
Grade Alignment
This Grade 12 Pre-Calculus worksheet supports work with discrete probability distributions by reinforcing the total-probability rule.
Students should already know how to check whether a distribution is valid. After this practice, they can move into translating probability phrases into notation.