MYP Standard Mathematics Topic Single-event…

Probability Starts With A Sample Space

A probability experiment is any repeatable situation with uncertain outcomes, for example tossing a coin, rolling a die, or picking a raffle ticket.
To talk about probability clearly, we first describe all possible outcomes.

A sample space can be written in different ways:

List: rolling a standard die: $S=\{1,2,3,4,5,6\}$
Table: sums from rolling two dice (each cell is an outcome)
Tree diagram: outcomes that happen in stages (first coin toss, then second toss, etc.)

The outcomes you include must match the experiment exactly.

Example

"Rolling two dice" needs outcomes like $(2,5)$ and $(5,2)$ separately (because they are different ordered outcomes), even though they give the same sum.

Common Mistake

A very common mistake is to forget outcomes or to treat outcomes as equally likely when they are not.
Always define the experiment first, then build the sample space.

Events Are Subsets Of The Sample Space

Once you have $S$, you can define what you care about.

Example

Given one die: $$S=\{1,2,3,4,5,6\}$$

Let $A$ be the event "getting a 3".
Then $A=\{3\}$.
The event "not getting a 3" is called the complement.
This implies $A'=\{1,2,4,5,6\}$.
Because either $A$ happens or it does not, $A \cup A' = S$.

The Single-Event Probability Formula

For single-event probability in a situation where outcomes are equally likely, we use: $$\mathrm{P}(A)=\frac{\text{number of outcomes in }A}{\text{number of outcomes in }S}=\frac{n(A)}{n(S)}$$
This is the key idea: probability is a proportion of the sample space.

Note

In real life, probability can also be interpreted as the proportion of times an event would occur over a very large number of trials.

Axioms You Must Always Obey

Probability has three basic rules (axioms):

Non-negativity: For any event $A$, $\mathrm{P}(A)\ge 0$.
Total probability: $\mathrm{P}(S)=1$.
Addition for mutually exclusive events: If events $A_1, A_2, \dots$ cannot happen together, then $$\mathrm{P}(A_1\cup A_2\cup \cdots)=\mathrm{P}(A_1)+\mathrm{P}(A_2)+\cdots$$

Using Complements To Calculate A Probability

The complement rule is extremely useful: $$\mathrm{P}(A)+\mathrm{P}(A')=1$$
So, $$\mathrm{P}(A')=1-\mathrm{P}(A)$$

Example

Given one die:

$$\mathrm{P}(\text{get a 3})=\frac{1}{6}$$

$$\mathrm{P}(\text{not get a 3})=1-\frac{1}{6}=\frac{5}{6}$$

Tip

If an event is described as "not …", try using the complement.
It can turn a hard count into an easy one.

Example

Building Sample Spaces In Common Formats

Lists For Simple Experiments

If you toss a coin once: $S=\{H,T\}$.
If you roll a 4-sided die: $S=\{1,2,3,4\}$.

Tables For Two-Stage Number Experiments

If you roll two 4-sided dice (faces $1$ to $4$), the sample space has $4\times 4=16$ equally likely ordered outcomes.
A table helps you see sums or products.
For example, the event "sum is 5" happens for $(1,4),(2,3),(3,2),(4,1)$, so: $$\mathrm{P}(\text{sum }=5)=\frac{4}{16}=\frac{1}{4}$$

Lists For Choice Experiments

If a street vendor sells two sandwiches (Gatsby, Gyro) and three fruits (apple, orange, banana), then there are $2\times 3=6$ outcomes: $$S=\{(\text{Gatsby, apple}), (\text{Gatsby, orange}), (\text{Gatsby, banana}), $$ $$(\text{Gyro, apple}), (\text{Gyro, orange}), (\text{Gyro, banana})\}$$
An event could be "choosing a banana", which has 2 outcomes out of 6, so probability $=\frac{2}{6}=\frac{1}{3}$.

Example

If 500 tickets are sold and exactly one ticket wins, and you buy $k$ tickets, then the sample space has 500 equally likely tickets.
Event $W$ = "you win".
There are $k$ winning possibilities for you (any of your tickets could be the winner), so: $$\mathrm{P}(W)=\frac{k}{500}$$
If you buy 10 tickets, $\mathrm{P}(W)=\frac{10}{500}=\frac{1}{50}=0.02$.
If you buy 0 tickets, $\mathrm{P}(W)=0$ (impossible).
If you buy all 500 tickets, $\mathrm{P}(W)=1$ (certain).

Active recall

A raffle has 200 tickets. You buy 15. What is the probability you win? What is the probability you do not win?

Tree Diagrams For Single Events In Multi-Step Games

A tree diagram is a structured way to list outcomes when an experiment happens in stages.
Each branch shows an outcome and its probability.

Example

The Two-Player Coin Game

Rules:

Peter flips first. If he gets Head, Peter wins. If Tail, Eliott flips.
Eliott flips. If Head, Eliott wins. If Tail, it returns to Peter.

This is a multi-step process, but each question like "Peter wins on his first turn" is still a single event (one event you are calculating).

(i) Probability Peter Wins On His First Turn

Peter wins immediately if his first flip is Head: $$\mathrm{P}(\text{Peter wins on first turn})=\frac{1}{2}$$

(ii) Probability Eliott Wins On His First Turn

For Eliott to win on his first turn, Peter must first flip Tail, then Eliott flips Head:

$$\mathrm{P}(T \text{ then } H)=\frac{1}{2}\times\frac{1}{2}=\frac{1}{4}$$

(iii) Probability Peter Wins On His Third Turn

Peter's third turn happens only if the first four flips are all tails (Peter T, Eliott T, Peter T, Eliott T), and then Peter flips Head on his third turn.

That path is: $T,T,T,T,H$.

$$\mathrm{P}(TTTTH)=\left(\frac{1}{2}\right)^5=\frac{1}{32}$$

Exam technique

On a tree diagram, multiply along a path (AND).
If you have several different winning paths, add the path probabilities (OR), but only if those paths are mutually exclusive.

Fairness And What "Equal Chance" Really Means

A fair game usually means each player has the same probability of winning.
In the coin game from the example above, Peter always goes first, so Peter has an advantage.
You can already see it because:
1. Peter can win immediately with probability $\frac{1}{2}$.
2. Eliott only gets a chance to win if Peter fails first.
If you continued the tree, Peter's overall chance of winning is greater than Eliott's.

Analogy

Going first in this game is like getting the first attempt at a multiple-choice question where the first correct answer ends the round.
Even if later players have the same chance per attempt, they get fewer opportunities before the round ends.

Note

Designing a fair game often means balancing "who goes first" with the rules, for example by changing the win condition or giving the second player a compensating advantage.

Simulation As A Reality Check

Sometimes you are told a probability (for example, "a winning envelope occurs 1 in 6 times").
One way to test how believable results are is to simulate.
If the chance is $\frac{1}{6}$, it matches "rolling a 6 on a fair die".
If 8 customers open envelopes, you can simulate by rolling a die 8 times and counting sixes.

Tip

Small samples can look surprising.
Getting 3 wins out of 8 does not automatically prove the claim is wrong, because random variation is expected in short runs.

Common Mistake

Not writing the sample space (or writing an incomplete one).
Assuming all outcomes are equally likely when they are not.
Adding probabilities when you should multiply (AND vs OR confusion).
Forgetting that $\mathrm{P}(A)+\mathrm{P}(A')=1$.

Active recall

Write the sample space for tossing a coin and rolling a die at the same time.
Find $\mathrm{P}(\text{tail and an even number})$.
Are the events "tail" and "even number" mutually exclusive? Explain.

Probability Starts With A Sample Space

A probability experiment is any repeatable situation with uncertain outcomes, for example tossing a coin, rolling a die, or picking a raffle ticket.
To talk about probability clearly, we first describe all possible outcomes.

A sample space can be written in different ways:

List: rolling a standard die: $S=\{1,2,3,4,5,6\}$
Table: sums from rolling two dice (each cell is an outcome)
Tree diagram: outcomes that happen in stages (first coin toss, then second toss, etc.)

The outcomes you include must match the experiment exactly.

Example

"Rolling two dice" needs outcomes like $(2,5)$ and $(5,2)$ separately (because they are different ordered outcomes), even though they give the same sum.

Common Mistake

A very common mistake is to forget outcomes or to treat outcomes as equally likely when they are not.
Always define the experiment first, then build the sample space.

Events Are Subsets Of The Sample Space

Once you have $S$, you can define what you care about.

Example

Given one die: $$S=\{1,2,3,4,5,6\}$$

Let $A$ be the event "getting a 3".
Then $A=\{3\}$.
The event "not getting a 3" is called the complement.
This implies $A'=\{1,2,4,5,6\}$.
Because either $A$ happens or it does not, $A \cup A' = S$.

The Single-Event Probability Formula

For single-event probability in a situation where outcomes are equally likely, we use: $$\mathrm{P}(A)=\frac{\text{number of outcomes in }A}{\text{number of outcomes in }S}=\frac{n(A)}{n(S)}$$
This is the key idea: probability is a proportion of the sample space.

Note

In real life, probability can also be interpreted as the proportion of times an event would occur over a very large number of trials.

Axioms You Must Always Obey

Probability has three basic rules (axioms):

Non-negativity: For any event $A$, $\mathrm{P}(A)\ge 0$.
Total probability: $\mathrm{P}(S)=1$.
Addition for mutually exclusive events: If events $A_1, A_2, \dots$ cannot happen together, then $$\mathrm{P}(A_1\cup A_2\cup \cdots)=\mathrm{P}(A_1)+\mathrm{P}(A_2)+\cdots$$

Using Complements To Calculate A Probability

The complement rule is extremely useful: $$\mathrm{P}(A)+\mathrm{P}(A')=1$$
So, $$\mathrm{P}(A')=1-\mathrm{P}(A)$$

Example

Given one die:

$$\mathrm{P}(\text{get a 3})=\frac{1}{6}$$

$$\mathrm{P}(\text{not get a 3})=1-\frac{1}{6}=\frac{5}{6}$$

Tip

If an event is described as "not …", try using the complement.
It can turn a hard count into an easy one.

Example

Building Sample Spaces In Common Formats

Lists For Simple Experiments

If you toss a coin once: $S=\{H,T\}$.
If you roll a 4-sided die: $S=\{1,2,3,4\}$.

Tables For Two-Stage Number Experiments

If you roll two 4-sided dice (faces $1$ to $4$), the sample space has $4\times 4=16$ equally likely ordered outcomes.
A table helps you see sums or products.
For example, the event "sum is 5" happens for $(1,4),(2,3),(3,2),(4,1)$, so: $$\mathrm{P}(\text{sum }=5)=\frac{4}{16}=\frac{1}{4}$$

Lists For Choice Experiments

If a street vendor sells two sandwiches (Gatsby, Gyro) and three fruits (apple, orange, banana), then there are $2\times 3=6$ outcomes: $$S=\{(\text{Gatsby, apple}), (\text{Gatsby, orange}), (\text{Gatsby, banana}), $$ $$(\text{Gyro, apple}), (\text{Gyro, orange}), (\text{Gyro, banana})\}$$
An event could be "choosing a banana", which has 2 outcomes out of 6, so probability $=\frac{2}{6}=\frac{1}{3}$.

Example

If 500 tickets are sold and exactly one ticket wins, and you buy $k$ tickets, then the sample space has 500 equally likely tickets.
Event $W$ = "you win".
There are $k$ winning possibilities for you (any of your tickets could be the winner), so: $$\mathrm{P}(W)=\frac{k}{500}$$
If you buy 10 tickets, $\mathrm{P}(W)=\frac{10}{500}=\frac{1}{50}=0.02$.
If you buy 0 tickets, $\mathrm{P}(W)=0$ (impossible).
If you buy all 500 tickets, $\mathrm{P}(W)=1$ (certain).

Active recall

A raffle has 200 tickets. You buy 15. What is the probability you win? What is the probability you do not win?

Tree Diagrams For Single Events In Multi-Step Games

A tree diagram is a structured way to list outcomes when an experiment happens in stages.
Each branch shows an outcome and its probability.

Example

The Two-Player Coin Game

Rules:

Peter flips first. If he gets Head, Peter wins. If Tail, Eliott flips.
Eliott flips. If Head, Eliott wins. If Tail, it returns to Peter.

This is a multi-step process, but each question like "Peter wins on his first turn" is still a single event (one event you are calculating).

(i) Probability Peter Wins On His First Turn

Peter wins immediately if his first flip is Head: $$\mathrm{P}(\text{Peter wins on first turn})=\frac{1}{2}$$

(ii) Probability Eliott Wins On His First Turn

For Eliott to win on his first turn, Peter must first flip Tail, then Eliott flips Head:

$$\mathrm{P}(T \text{ then } H)=\frac{1}{2}\times\frac{1}{2}=\frac{1}{4}$$

(iii) Probability Peter Wins On His Third Turn

Peter's third turn happens only if the first four flips are all tails (Peter T, Eliott T, Peter T, Eliott T), and then Peter flips Head on his third turn.

That path is: $T,T,T,T,H$.

$$\mathrm{P}(TTTTH)=\left(\frac{1}{2}\right)^5=\frac{1}{32}$$

Exam technique

On a tree diagram, multiply along a path (AND).
If you have several different winning paths, add the path probabilities (OR), but only if those paths are mutually exclusive.

Fairness And What "Equal Chance" Really Means

A fair game usually means each player has the same probability of winning.
In the coin game from the example above, Peter always goes first, so Peter has an advantage.
You can already see it because:
1. Peter can win immediately with probability $\frac{1}{2}$.
2. Eliott only gets a chance to win if Peter fails first.
If you continued the tree, Peter's overall chance of winning is greater than Eliott's.

Analogy

Going first in this game is like getting the first attempt at a multiple-choice question where the first correct answer ends the round.
Even if later players have the same chance per attempt, they get fewer opportunities before the round ends.

Note

Designing a fair game often means balancing "who goes first" with the rules, for example by changing the win condition or giving the second player a compensating advantage.

Simulation As A Reality Check

Sometimes you are told a probability (for example, "a winning envelope occurs 1 in 6 times").
One way to test how believable results are is to simulate.
If the chance is $\frac{1}{6}$, it matches "rolling a 6 on a fair die".
If 8 customers open envelopes, you can simulate by rolling a die 8 times and counting sixes.

Tip

Small samples can look surprising.
Getting 3 wins out of 8 does not automatically prove the claim is wrong, because random variation is expected in short runs.

Common Mistake

Not writing the sample space (or writing an incomplete one).
Assuming all outcomes are equally likely when they are not.
Adding probabilities when you should multiply (AND vs OR confusion).
Forgetting that $\mathrm{P}(A)+\mathrm{P}(A')=1$.

Active recall

Write the sample space for tossing a coin and rolling a die at the same time.
Find $\mathrm{P}(\text{tail and an even number})$.
Are the events "tail" and "even number" mutually exclusive? Explain.

MYP Standard Mathematics Single-event Probability Notes

MYP Standard Mathematics Single-event Probability Notes

Probability Starts With A Sample Space

Events Are Subsets Of The Sample Space

The Single-Event Probability Formula

Axioms You Must Always Obey

Using Complements To Calculate A Probability

Building Sample Spaces In Common Formats

Lists For Simple Experiments

Tables For Two-Stage Number Experiments

Lists For Choice Experiments

Tree Diagrams For Single Events In Multi-Step Games

The Two-Player Coin Game

Fairness And What "Equal Chance" Really Means

Simulation As A Reality Check

Probability Starts With A Sample Space

Events Are Subsets Of The Sample Space

The Single-Event Probability Formula

Axioms You Must Always Obey

Using Complements To Calculate A Probability

Building Sample Spaces In Common Formats

Lists For Simple Experiments

Tables For Two-Stage Number Experiments

Lists For Choice Experiments

Tree Diagrams For Single Events In Multi-Step Games

The Two-Player Coin Game

Fairness And What "Equal Chance" Really Means

Simulation As A Reality Check

Number and Algebra Foundations

Coordinate Geometry and Linear Modelling

Functions and Quadratic Relationships

Sets, Logic and Probability

Geometry of Shapes, Surface Area and Volume

Statistical Measures and Data Representation

Right Triangles and Circle Geometry

Sequences, Formulae and Proportional Reasoning

Sampling Methods and Bivariate Data

MYP Standard Mathematics Single-event Probability Notes

Number and Algebra Foundations

Coordinate Geometry and Linear Modelling

Functions and Quadratic Relationships

Sets, Logic and Probability

Geometry of Shapes, Surface Area and Volume

Statistical Measures and Data Representation

Right Triangles and Circle Geometry

Sequences, Formulae and Proportional Reasoning

Sampling Methods and Bivariate Data

MYP Standard Mathematics Single-event Probability Notes

Probability Starts With A Sample Space

Events Are Subsets Of The Sample Space

The Single-Event Probability Formula

Axioms You Must Always Obey

Using Complements To Calculate A Probability

Building Sample Spaces In Common Formats

Lists For Simple Experiments

Tables For Two-Stage Number Experiments

Lists For Choice Experiments

Tree Diagrams For Single Events In Multi-Step Games

The Two-Player Coin Game

Fairness And What "Equal Chance" Really Means

Simulation As A Reality Check

Probability Starts With A Sample Space

Events Are Subsets Of The Sample Space

The Single-Event Probability Formula

Axioms You Must Always Obey

Using Complements To Calculate A Probability

Building Sample Spaces In Common Formats

Lists For Simple Experiments

Tables For Two-Stage Number Experiments

Lists For Choice Experiments

Tree Diagrams For Single Events In Multi-Step Games

The Two-Player Coin Game

Fairness And What "Equal Chance" Really Means

Simulation As A Reality Check