PSLE Math Revision: Percentage

• % just means “out of 100.” Simple, right?
• Convert between % ↔ decimals ↔ fractions like a boss.
• Find percentage of numbers, increases, decreases.

🧠 Formula: (Part / Whole) × 100%

🧩 Percentage Word Problem Concepts

1. One Item Constant

One value doesn’t change – e.g., same number of items.

🔍 Example: Susan has 25% more stickers than Amanda. Mary has 25% fewer stickers than Amanda. Susan has 200 more stickers than Mary. How many stickers does Susan and Amanda have altogether?

Ans: 900

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2. Constant Total

The total stays the same – e.g., marks, money, number of people.

🔍 Example: Cathy bought a novel. The number of pages of novel she had read last week was 20%. If she read another 240 pages, the number of pages she had not read would become 20%. How many pages did her novel have?

Ans: 400

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3. Constant Difference

The gap between two values stays the same before and after a change.

🔍 Example: There was a jar of blue and green buttons. 60% of the buttons were blue and the rest were green. After adding another 140 blue and 140 green buttons, the ratio of the blue buttons to green buttons became 7 : 5. How many buttons were there in the jar in the end?

Ans: 1680

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4. More Than / Less Than

Used when comparing two values with “__% more/less than” language.

🔍 Example: A is 25% more than B → A = 1.25B

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5. Equal Percentage

Different amounts can have the same percentage change.

🔍 Example: Two stores both offer 20% discount. One reduces $60, another $40 → both used same rate.

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6. Part-Whole Relationship

Know what the % refers to: part or whole?

🔍 Example: 30% of students = 60 → 100% = ? → 60 ÷ 30 × 100 = 200

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7. Repeated Identity

The same unknown appears in multiple places.

🔍 Example: 20% of A = 40% of B → Link both unknowns using equations.

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8. External (Unchanged Total Quantity)

Change happens within a group, but the total stays the same.

🔍 Example: Boys increased by 20%, girls decreased by 10%, but total students = same.

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9. Units and Parts

Convert units when necessary (e.g., grams to kg, minutes to hours) before calculating %.

🔍 Example: 250g is what % of 2kg? → 250 ÷ 2000 × 100 = 12.5%

✅ Always match the units before solving!