Rule of Three Calculator

How to use it

1. {'step': 1, 'description': 'Set up proportion: 7.5 is to 22.5 as 12.8 is to x'}
2. {'step': 2, 'description': 'Cross multiply: x = (22.5 × 12.8) ÷ 7.5'}
3. {'step': 3, 'description': 'Calculate numerator: 22.5 × 12.8 = 288'}
4. {'step': 4, 'description': 'Divide: 288 ÷ 7.5 = 38.4 (cost of 12.8kg)'}

Common mistakes

• ⚠️ Setting up the proportion incorrectly (wrong position of values)
• ⚠️ Not ensuring the relationship is truly proportional before applying
• ⚠️ Dividing by a when a equals zero, causing calculation error

Worked example

Calculate: if 5 items cost 25, what cost 8?

The result x ({x}) is the unknown fourth value in the proportion a:b :: c:x, solving for equivalent ratios in direct proportion problems.

Input guide