Work out percentages three ways: what is X% of Y, X is what % of Y, and the percentage change from A to B.
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Divide the percentage by 100 and multiply by the number — 20% of 150 is 20 / 100 × 150 = 30.
Subtract the old value from the new, divide by the old value and multiply by 100. Positive is an increase, negative a decrease.