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A scientist measures the initial amount of Carbon$-14$ in a substance to be $25$ grams.$\newline$The relationship between $\newline$$A$, the amount of Carbon$-14$ remaining in that substance, in grams, and $\newline$$t$, the elapsed time, in years, since the initial measurement is modeled by the following equation.$\newline$$$A=25e^{-0.00012 t}$$$\newline$In how many years will the substance contain exactly $20$ grams $\newline$(g) of Carbon$14$ ?$\newline$Give an exact answer expressed as a natural logarithm.$\newline$years