Find a percentage, reverse it, compare change, and avoid the margin-versus-markup trap.
Part of a number
To find X% of Y, multiply Y by X/100. A 15% tip on 80 is 80 × 0.15 = 12. The total paid is a separate step: 80 + 12 = 92.
Percentage change
Change uses the original value as the denominator: (new − old) / old × 100. A rise from 50 to 60 is 20%, while the return from 60 to 50 is a 16.67% fall. Equal absolute changes do not imply equal percentage changes.
Margin is not markup
Margin divides profit by selling price; markup divides profit by cost. If cost is 80 and selling price is 100, profit is 20, margin is 20%, and markup is 25%. Choose the denominator that matches your business question.