This study notes covers Worked Example 4: Finding Missing Value within Ratio Basics for GCSE Mathematics. Revise Ratio Basics in Number for GCSE Mathematics with 12 exam-style questions and 22 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 10 of 15 in this topic. Use this study notes to connect the idea to the wider topic before moving on to questions and flashcards.
Worked Example 4: Finding Missing Value
If the ratio of boys to girls is 4:5 and there are 20 boys, how many girls are there?
Solution
Boys : Girls = 4 : 5
20 : ? = 4 : 5
Scale factor: 20 ÷ 4 = 5
Number of girls = 5 × 5 = 25
Answer: 25 girls
Practice questions for Ratio Basics
Write the ratio 12:18 in its simplest form.
Explain the difference between the ratio 3:5 and the ratio 5:3.
Quick recall flashcards
What is a ratio?
A comparison of quantities of the same kind
Written as a:b or a/b
Example: 3:5 means '3 parts to 5 parts'
Order in Ratios
Order MATTERS!
Boys:Girls = 3:5 means:
• 3 boys for every 5 girls
Girls:Boys = 5:3 means:
• 5 girls for every 3 boys
Different meanings!