In order to answer the question, we have to solve the equation:
[tex]3e^{-x}\ \textless \ 0.055,\text{ divide by 3 like this:}\\e^{-x}\ \textless \ \frac{0.055}{3}\\e^{-x}\ \textless \ 0.018,\text{ apply the log function to the two sides of the inequality:}\\-x\ \textless \ \log(0.018)\\-x\ \textless \ -4,\text{ therefore:}\\x\ \textgreater \ 4[/tex]
So it take 4 years in order to the kryptonite to be less than 0.055 grams.
