Percentage formulas
A percentage of an amount is amount × rate ÷ 100. Subtract this share for a discount or add it for an increase. Percentage change is (final − starting amount) ÷ starting amount × 100. A zero starting amount cannot define percentage change.
Examples and common pitfalls
A 15% discount on CAD 80 saves CAD 12 and leaves CAD 68. A change from 100 to 120 is a 20% increase. Successive discounts compound: 20% then 10% off CAD 100 leaves CAD 72, not CAD 70. A 20% decrease followed by a 20% increase leaves CAD 96.
Sales tax is not included. Displayed money is rounded to cents; intermediate arithmetic retains precision.
Source: Statistics Canada: basic statistical techniques (French source).