Worked Example 1: Basic Direct Proportion

Part of Proportion Graphs · Section 4 of 6

Study NotesUnit: Ratio & ProportionGCSE

y is directly proportional to x. When x = 5, y = 20. Find y when x = 8.

Step 1 Set up the equation

y ∝ x means y = kx

Step 2 Find k using given values

20 = k × 5

k = 20 ÷ 5 = 4

Step 3 Find y when x = 8

y = 4 × 8 = 32

This study notes covers Worked Example 1: Basic Direct Proportion within Proportion Graphs for GCSE Mathematics. Revise Proportion Graphs in Ratio & Proportion for GCSE Mathematics with 14 exam-style questions and 4 flashcards. Use this page as part of a wider topic revision path rather than treating it as an isolated fact. It is section 4 of 6 in this topic. Use this study notes to connect the idea to the wider topic before moving on to questions and flashcards.

Practice questions for Proportion Graphs

Which of the following describes the graph of a direct proportion relationship?

A. A curved line passing through the origin
B. A straight line passing through the origin
C. A straight line that crosses the y-axis above zero
D. A U-shaped curve symmetric about the y-axis

1 markfoundation

Aisha says: 'Because the graph of y against x is a straight line, y must be directly proportional to x.' Is Aisha correct? Explain your answer.

2 marksstandard

Quick recall flashcards

Direct vs Inverse

Direct: both increase together (y ∝ x). Inverse: one increases, other decreases (y ∝ 1/x).

Finding k

Use given values to find constant k, then apply to new values

Method: Finding the Constant

Worked Example 2 (Higher): Proportional to Square

14 questions on Proportion Graphs: practise free

Instant marking, adaptive difficulty and spaced-repetition flashcards — all aligned to your exam board.

Start revising free →