Percentage of a number: multiply by the percentage and divide by 100. A 20% discount on $80 leaves $64. A rise from 250 to 310 is a 24% increase.
1. A discount
Price × (1 − discount). For 20% off $80: 80 × 0.8 = $64.
2. A tip, split
Bill × tip rate. A tip of 18% on an $86.40 bill is $15.55, a total of $101.95, or $50.98 each between two people.
3. Sales tax
Price × tax rate. At 8.75% on $120, the tax is $10.50.
4. Percentage change
(New − old) ÷ old × 100. From 250 to 310 the change is 60, which is 24% of 250.
5. A share of a total
Part ÷ total × 100. 45 out of 180 is 25%.
FAQ
Is a 10% rise followed by a 10% fall back to the start?+
No. The fall applies to a larger number, so you end 1% below the start.
How do I reverse a percentage change?+
Divide by (1 + rate) rather than subtracting the rate.