Percentages — Fast Mental Math

Percent means per hundred. 35% = 35/100 = 0.35

Finding a percent: 20% of 150 = 0.20 x 150 = 30

Percent increase: (New - Old) / Old x 100

$80 to $100: (100 - 80)/80 x 100 = 25% increase

Percent decrease: (Old - New) / Old x 100

$100 to $80: (100 - 80)/100 x 100 = 20% decrease

CRITICAL: 25% increase does NOT equal 25% decrease of the result.

$100 + 25% = $125. But $125 - 25% = $93.75, NOT $100.

Successive percentages: Multiply the multipliers.

20% increase then 10% decrease: 1.20 x 0.90 = 1.08 = 8% net increase

Reference:

Wikipedia: Percentage

https://en.wikipedia.org/wiki/Percentage