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Year 7 · Maths · Primes, factors and probability · Lesson 4

Probability

Use the probability scale and words, calculate probabilities of equally likely outcomes as fractions, and use P(not A) = 1 − P(A).

Whiteboard video lesson: Probability

27-minute video lesson

Teaching, worked examples and 20 practice questions solved step by step.

Watch free for a month

What your child will learn

  • Use the probability scale
  • Describe likelihood in words
  • Calculate simple probabilities
  • Use P(not A) = 1 − P(A)
  • Know probabilities add to 1

The method

  1. Count all the equally likely outcomes.
  2. Count the outcomes you want.
  3. P = wanted ÷ total; simplify.
  4. P(not) = 1 − P; all probabilities add to 1.

Key words: probability, outcome, equally likely, certain, impossible.

Worked examples

The first three practice questions from the lesson, with every step.

Question 1

A fair dice is rolled. Find P(2).

  1. Step 1: Outcomes: 6
  2. Step 2: A 2: 1
  3. Step 3: P(a 2) = 1/6
  4. Step 4: Can 1/6 simplify? No.

Answer: 1/6

Question 2

A fair dice is rolled. Find P(odd number).

  1. Step 1: Outcomes: 6
  2. Step 2: Odd: 3
  3. Step 3: P(odd) = 3/6
  4. Step 4: 3/6 = 1/2 (÷ 3)

Answer: 1/2

Question 3

A fair dice is rolled. Find P(more than 4).

  1. Step 1: Outcomes: 6
  2. Step 2: More than 4: 2
  3. Step 3: P(more than 4) = 2/6
  4. Step 4: 2/6 = 1/3 (÷ 2)

Answer: 1/3