Common percentage formulas
- B% of A = A × B ÷ 100 (e.g. 15% of ₩50,000 = ₩7,500)
- A is what % of B = A ÷ B × 100 (e.g. 30 points is 25% of 120)
- Change rate = (new value − old value) ÷ old value × 100
- Sale price = list price × (1 − discount rate ÷ 100)
% vs. %p (percentage points)
If a rate rises from 3% to 4%, that's "up 1%p", but a 33.3% increase. These are often confused in the news, so keep them apart.
Stacked discounts
20% off and then another 10% off is not 30% off but 28% off (0.8 × 0.9 = 0.72).
FAQ
If something rises 10% and then falls 10%, is it back where it started?
No. 100 → 110 → 99, so it's down 1%.