This study notes covers Worked Example 1: Direct Proportion within Proportion for GCSE Mathematics. Revise Proportion in Number for GCSE Mathematics with 12 exam-style questions and 22 flashcards. Use this page as part of a wider topic revision path rather than treating it as an isolated fact. It is section 7 of 16 in this topic. Use this study notes to connect the idea to the wider topic before moving on to questions and flashcards.
Worked Example 1: Direct Proportion
y is directly proportional to x. When x = 5, y = 20. Find the value of y when x = 8.
Solution
y ∝ x, so y = kx
When x = 5, y = 20: 20 = k × 5
k = 20 ÷ 5 = 4
So y = 4x
When x = 8: y = 4 × 8 = 32
Answer: y = 32
Practice questions for Proportion
y is directly proportional to x. Which equation could represent this relationship?
Explain how you can tell from a graph whether two quantities are in direct or inverse proportion.