🔢 Rule of Three 📚 Grade 4-5 ⏱ 6 min
Rule of Three for Solving Proportions
The rule of three is a simple way to find an unknown number knowing the relationship between three others. Here it is in a clear example.
A problem with a known ratio
3 apples cost $6. How much do 5 apples cost? Rule of three: (6÷3)×5=$10.
💡 The logic: first find the price of one apple (6÷3=2), then multiply by the needed quantity (2×5=10).
Practice
4 notebooks cost $8. How much do 7 notebooks cost? (8÷4)×7=$14.
FAQ
Where does the rule of three show up in life?
Price calculations, scaling recipes to a different serving count, resizing any proportional quantity.