Need to Know

Probability

The rules you must have ready for the exam.

How to use this: say each rule in your head, then tap Show me to check.

The basics

1 rule

Rule 01

Formula & scale

P(E) = desired ÷ total

Scale: 0 impossible → 1 certain. AND = multiply, OR = add.

1 rule

Rule 02

List the outcomes

Write out every equally-likely outcome (or a grid for two dice), then count the ones you want.

1 rule

Rule 03

Multiply the stages

Options at each stage multiply: 3 × 4 × 2 × 3 = 72.

1 rule

Rule 04

Along and between

Multiply along the branches of one path; add the paths that satisfy the event.

1 rule

Rule 05

From data

Relative frequency = result ÷ trials. All of them add to 1.

Mathslive.ie · Probability · Need to Know