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Q&A Why must percent change divide the difference by the old, NOT new, value?

1 answer · posted 3y ago by Chgg Clou‭ · last activity 3y ago by celtschk‭

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#1: Initial revision by $user avatar$ Chgg Clou‭ · 2021-12-19T20:01:25Z (almost 3 years ago)

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Why must percent change  divide the difference by the old, NOT new, value?

![](https://www.onlinemathlearning.com/image-files/percent-change.png)

My 14 year old still cannot intuit why the denominator must be the old number, not the new — not even after reading the answers on [Math SE]( 
 https://math.stackexchange.com/q/2896701). Can someone explain this better?

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