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Explanation:
If you lose 9% of a substance, then you keep the remaining 91% because 100% - 9% = 91%. Or you can think of it as 91% + 9% = 100%
Then we convert 91% to decimal form to get 0.91
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Or you can do the steps like this:
9% = 0.09
1 - 0.09 = 0.91