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\begin{array}{ll}t=\frac{\ln 2}{k} & \text { The doubling time formula. } \\2=\frac{\ln 2}{k} & \text { Use a doubling time of two years. } \\k=\frac{\ln 2}{2} & \text { Multiply by } k \text { and divide by } 2 . \\A=A_{0} e^{\frac{\ln 2}{2} t} & \text { Substitute } k \text { into the continuous growth formula. }\end{array}