Question 292269
Why is $10 to $20 a 100% increase, while $20 to $10 is a 50% decrease. Explain your answer.
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It's based on what the change is from.
From 10 to 20 starts with 10.  10/10 is 100%
From 20 to 10 starts with 20.  10/20 is 50%
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What you start with is the base.  
If you try to do it any other way, it's a lot less meaningful.