Ratio, Proportion and Rates Of Change — OCR GCSE Study Guide

 ## Overview Welcome to one of the most critical topics in your GCSE Mathematics specification: Ratio, Proportion, and Rates of Change. This section focuses specifically on the holy trinity of numbers—fractions, decimals, and percentages. Why does this matter? Because examiners love to test your fluency in moving between these forms. It's not just about answering dedicated percentage questions; these skills are synoptic. You will need them for compound interest, probability, pie charts, and even algebraic fractions. Typically, questions on this topic range from straightforward 1-mark conversions to complex 5-mark problem-solving scenarios involving repeated percentage change. The difference between a Grade 5 and a Grade 7 often comes down to one thing: using percentage multipliers efficiently instead of relying on slow, multi-step addition and subtraction methods. Listen to the companion podcast for this topic below:  ## Key Concepts ### Concept 1: The Three Languages of Number Fractions, decimals, and percentages are simply three different ways of writing the exact same value. Think of them as English, French, and Spanish—different words, same meaning. Examiners test your ability to translate between them seamlessly.  - **Fractions to Decimals**: Divide the numerator (top) by the denominator (bottom). For example, $3 \div 4 = 0.75$. - **Decimals to Percentages**: Multiply by 100. For example, $0.75 \times 100 = 75\%$. - **Percentages to Fractions**: Write the percentage over 100 and simplify. For example, $75\%$ becomes $\frac{75}{100}$, which simplifies to $\frac{3}{4}$. **Examiner Tip**: A common error is converting $5\%$ to a decimal. Many candidates write $0.5$ (which is $50\%$). Remember, $5 \div 100 = 0.05$. ### Concept 2: Ordering Mixed Types When faced with a list of numbers in different formats (e.g., $\frac{2}{5}$, $0.45$, $42\%$), the golden rule is: **convert everything to decimals first**. Decimals are the easiest format to compare because of clear place value. **Example**: Order $\frac{3}{8}$, $0.35$, and $38\%$ from smallest to largest. 1. Convert $\frac{3}{8}$: $3 \div 8 = 0.375$ 2. Convert $38\%$ to a decimal: $0.38$ 3. We now have: $0.375$, $0.35$, and $0.38$. 4. Ordering them: $0.35$, $0.375$, $0.38$. 5. Final answer in original format: $0.35$, $\frac{3}{8}$, $38\%$. ### Concept 3: Percentage Multipliers (The Game Changer) This is the most powerful tool in your GCSE Maths toolkit. A multiplier is a single decimal number that applies a percentage change in one swift calculation.  - **For an Increase**: Add the percentage to $100\%$, then convert to a decimal. (e.g., a $15\%$ increase means you have $115\%$, so the multiplier is $1.15$). - **For a Decrease**: Subtract the percentage from $100\%$, then convert to a decimal. (e.g., a $20\%$ decrease means you have $80\%$ left, so the multiplier is $0.80$). **Why this works**: Instead of finding the percentage and then adding or subtracting it (a two-step process where errors often occur), you scale the original amount directly. ### Concept 4: Reverse Percentages This is where many candidates drop marks. A reverse percentage question gives you the *new* amount after a change and asks for the *original* amount. You **cannot** simply apply the percentage change in reverse. You must use algebraic thinking or inverse operations with multipliers. **Example**: A shirt is on sale for £34 after a $15\%$ discount. What was the original price? 1. Let the original price be $x$. 2. The multiplier for a $15\%$ decrease is $0.85$. 3. Therefore, $x \times 0.85 = 34$. 4. To find $x$, divide: $x = 34 \div 0.85 = 40$. 5. The original price was £40. ## Mathematical Relationships - **Multiplier Formula**: $\text{Multiplier} = 1 \pm \left(\frac{\text{Percentage Change}}{100}\right)$ - **Percentage Change Formula**: $\frac{\text{Change}}{\text{Original}} \times 100$ - **Compound Interest Formula**: $\text{Final Amount} = \text{Initial Amount} \times \text{Multiplier}^{\text{Years}}$ ## Practical Applications These skills are used daily in the real world. Calculating VAT on a purchase, determining the best mortgage rate, figuring out if a "Buy One Get One Half Price" deal is better than "30% Off", or calculating the depreciation of a car's value over time all rely on mastering fractions, decimals, and percentages.