If a price after a 20% increase is 120, the original price was 100: divide 120 by 1.20. Subtracting 20% of 120 produces 96 because it uses the new price as its base.
Why this works
For an increase of p%, divide the new value by 1 + p/100. For a reduction of p%, divide by 1 − p/100. A 100% reduction cannot be reversed from the final value alone.
When this answer changes
Forward changes multiply by a factor; reversing changes divides by that same factor. A 20% increase is reversed by a 16.6667% decrease, because 20 divided by 120 is one sixth.
Try your own values
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