To find X% of a number, enter the number × X ÷ 100
Example: 25% of 80 = 80 × 25 ÷ 100
To find what percent one number is of another, enter the part ÷ whole × 100
Example: 20 is what percent of 80 = 20 ÷ 80 × 100
To increase a number by X%, enter the number × (100 + X) ÷ 100
Example: increase 50 by 10% = 50 × 110 ÷ 100
To decrease a number by X%, enter the number × (100 – X) ÷ 100
Example: decrease 50 by 10% = 50 × 90 ÷ 100
To find the percentage change, enter (new – old) ÷ old × 100
Example: from 40 to 50 = (50 – 40) ÷ 40 × 100
If your calculator has a % key, enter the number, then the percent, then press %
Check the calculator result against the formula if needed