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Three common percentage questions
What is X% of a number? Multiply the number by X and divide by 100. What percentage is one number of another? Divide the part by the whole and multiply by 100. What is the percentage change? Subtract the original value from the new value, divide by the original value and multiply by 100.
Choose the correct base value
Percentage results depend on the reference amount. A change from 80 to 100 is a 25% increase, because the difference of 20 is compared with the original 80. A change from 100 back to 80 is a 20% decrease, because the same difference is compared with 100.
When the original or base value is zero, percentage change is undefined. Check which number is the starting value before interpreting the result.