📖Definition
Probability measures how likely an event is to happen. It is expressed as a number between 0 (impossible) and 1 (certain), or as a percentage (0% to 100%). Probability is calculated by dividing favorable outcomes by total possible outcomes.
📐Formula
Probability ranges from 0 to 1. P = 0 means impossible, P = 1 means certain. For a fair coin flip, P(heads) = 1/2 = 0.5 = 50%.
📝Step-by-Step Guide
Identify the Event
Clearly define what outcome(s) you're calculating the probability for.
Count Favorable Outcomes
Determine how many outcomes match your event.
Count Total Possible Outcomes
List or count all possible outcomes in the sample space.
Calculate the Probability
Divide favorable by total and simplify or convert to percentage.
⚠️Common Mistakes to Avoid
- Confusing "at least one" with "exactly one"
- Forgetting probability must be between 0 and 1
- Not counting all possible outcomes
- Treating non-equally-likely outcomes as equal
- Confusing independent and dependent events
✏️Practice Problems
What is the probability of rolling a 4 on a standard die?
Answer: 1/6
A bag has 3 red and 5 blue marbles. What is the probability of picking a red marble?
Answer: 3/8
What is the probability of rolling an even number OR a number greater than 4 on a die?
Answer: 4/6 = 2/3
Ready to Practice Probability?
Generate a personalized practice test with MathQuizily. Get instant PDF downloads with answer keys.