1
Write y = kx (y is directly proportional to x)
2
Use given values to find k
3
Substitute k back into equation
4
Use equation to find unknown values
5
Example: If y ∝ x and y = 12 when x = 3, find k = 12/3 = 4, so y = 4x
This deep dive covers Method: 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 4 of 16 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
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.
Quick recall flashcards
Recipe Application
Recipe for 4 people uses 300g flour
How much for 10 people?
Flour ∝ People (direct)
F = kP
300 = k × 4, so k = 75
For 10 people: F = 75 × 10 = 750g
Proportion Symbol
∝ means 'proportional to'
Examples:
y ∝ x (y is proportional to x)
y ∝ 1/x (y is inversely proportional to x)