TeX source:
\begin{array}{rclcl} u_1 = 5^{t_1} &=& 0 \cr\cr u_2 = 5^{t_2} &=& 16 &\vert&\ln() \cr \ln(5^t_2) &=& \ln(16) &\vert& \text{3. Logarithmengesetz} \cr t_2 \cdot \ln(5) &=& \ln(16) &\vert&:\ln(5) \cr t_2 &=& \dfrac{\ln(16)}{\ln(5)} \approx1{,}72 \end{array}