Percentages run daily life — discounts, tips, interest, pay rises — yet most adults reach for a phone. Three shortcuts cover almost every real-world case.
1. The 10% Anchor
10% = move the decimal point. Build everything from there: 5% is half of it, 1% is a tenth of it, 20% is double. So 35% of £80: 10% is £8 → 30% is £24 → 5% is £4 → £28. Two seconds, no phone.
2. The Flip Trick
a% of b always equals b% of a. Awkward 8% of 25? Flip it: 25% of 8 = 2. 4% of 75 = 75% of 4 = 3. When one direction is ugly, the other is usually clean.
3. Reverse Percentages (The One Everyone Gets Wrong)
A £60 jumper after a 20% discount did not cost £72 originally. The £60 is 80% of the original, so divide: £60 ÷ 0.8 = £75. Same for VAT: a £120 gross invoice contains £20 of VAT (÷1.2), not £24. Adding the percentage back never undoes taking it off.
Bonus: Percentage Change
(New − old) ÷ old × 100. From £2,000 to £2,300 rent is +15%. And sequential changes multiply, not add: +50% then −50% leaves you at 75%, not evens — the reason volatile investments quietly bleed.