To find a percentage of a number, multiply the number by the percentage and divide by 100
Example: 20% of 150 = 150 × 20 ÷ 100
To add a percentage, multiply the number by 1 plus the percentage as a decimal
Example: 150 + 20% = 150 × 1.20
To subtract a percentage, multiply the number by 1 minus the percentage as a decimal
Example: 150 – 20% = 150 × 0.80
To find what percent one number is of another, divide the first number by the second and multiply by 100
Example: 30 is what percent of 150 = 30 ÷ 150 × 100
To find the percentage increase, subtract the old value from the new value, divide by the old value, then multiply by 100
Example: (180 – 150) ÷ 150 × 100
To find the percentage decrease, subtract the new value from the old value, divide by the old value, then multiply by 100
Example: (150 – 120) ÷ 150 × 100
On many calculators, use the percent key by entering the number, pressing the operation, entering the percentage, then pressing %
If your calculator has no percent key, use ÷ 100 for percentage calculations