[tex]\bf r=100\left( \frac{1}{2} \right)^{\frac{d}{5}}\\\\
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\textit{after 1 day how much remains?}\qquad \boxed{d=1}\qquad r=100\left( \frac{1}{2} \right)^{\frac{1}{5}}
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r=100\left( \cfrac{1^{\frac{1}{5}}}{2^{\frac{1}{5}}} \right)\implies r=100\left( \cfrac{1}{\sqrt[5]{2}} \right)\implies r\approx 87.06
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\textit{used to be 100mg, it went down in 1 day to 87.06 or }12.94[/tex]
how much is 12.94 in percent of 100? well, since is 100, is just 12.94%.
