Probability 1 (Edexcel GCSE Maths): Revision Notes
Probability 1
What is probability?
Probability measures how likely an event is to happen. It gives us a numerical way to describe the chance of something occurring.
The probability of any event is always a value between 0 and 1, where:
- 0 means impossible - the event will never happen
- 1 means certain - the event will definitely happen
- Values between 0 and 1 show different levels of likelihood
The probability scale
The probability scale runs from 0 to 1 and helps us understand how likely events are:
- Impossible (0) → Even chance (0.5) → Certain (1)
At the halfway point (0.5), an event has an even chance of happening - it's equally likely to happen or not happen.
Writing probabilities
You can express the same probability in three different ways:
- Fraction:
- Decimal: 0.5
- Percentage: 50%
All three represent the same likelihood - an even chance.
Golden rule for calculating probability
The Golden Rule Formula:
This formula helps you calculate exact probabilities when you know all the possible outcomes.
Examples using the golden rule
Worked Example: Rolling a Dice
To find P(rolling a 6):
- Number of successful outcomes = 1 (only one way to roll a 6)
- Total possible outcomes = 6 (faces 1, 2, 3, 4, 5, 6)
- P(6) =
Worked Example: Flipping a Coin
To find P(heads):
- Number of successful outcomes = 1 (only one way to get heads)
- Total possible outcomes = 2 (heads or tails)
- P(heads) =
Worked Example: Spinning a Spinner
If a spinner has 8 equal sections and 4 sections show the letter X:
- Number of successful outcomes = 4 (sections with X)
- Total possible outcomes = 8 (all sections)
- P(landing on X) =
Probability language
Use these words to describe different levels of probability:
- Impossible - will never happen (probability = 0)
- Unlikely - probably won't happen (probability less than 0.5)
- Even - equally likely to happen or not (probability = 0.5)
- Likely - probably will happen (probability greater than 0.5)
- Certain - will definitely happen (probability = 1)
Writing probability notation
When writing probabilities mathematically, use this format:
- P(event) = probability value
- Example: P(heads) =
This notation clearly shows what event you're calculating the probability for.
Exam tips
Essential Exam Strategies:
- Always check that your probability answers are between 0 and 1
- Remember that probabilities can be written as fractions, decimals or percentages
- Count outcomes carefully when using the golden rule
- Use appropriate probability words to describe likelihood in exam questions
Key Points to Remember:
- Probability measures how likely events are to happen, using values from 0 to 1
- Use the golden rule: Probability =
- Probabilities can be written as fractions, decimals or percentages
- Learn probability vocabulary: impossible, unlikely, even, likely, certain
- Always check your answers make sense - probabilities must be between 0 and 1