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What Is the Probability?(With Examples)

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.

Grades 7-8Key Stage 3 (Year 7-9)Årskurs 7-9Klasse 7-8

📖Definition

📐Formula

P(Event) = Number of favorable outcomes / Total number of possible outcomes

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.

P(Event) = Favorable / Total

⚠️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

Easy

What is the probability of rolling a 4 on a standard die?

Answer: 1/6

Medium

A bag has 3 red and 5 blue marbles. What is the probability of picking a red marble?

Answer: 3/8

Hard

What is the probability of rolling an even number OR a number greater than 4 on a die?

Answer: 4/6 = 2/3

Printable Probability practice

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Worked Examples

Probability Examplemedium

Statistics Fractions Percentages Combinations

Curriculum Alignment

CommonCore (7.SP.C.5)KS3KMK

Classroom assessment context

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