Percentage
A number expressed as a fraction of 100, used to describe proportions, changes, and comparisons.
Percentage change: (New - Old) / Old × 100. Percentage of: X% of Y = X/100 × Y. Key insight: X% of Y equals Y% of X (8% of 50 = 50% of 8 = 4). Going from 100 to 80 is a 20% decrease. Going from 80 back to 100 is a 25% increase — the asymmetry trips up investors because the same absolute change is a different percentage depending on the starting value.
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