75% of a first order reaction is completed in 30 minutes, what is the time required for 93.75% of the reaction in minutes?

$\begin{array}{l}\mathrm{k}=\frac{2.303}{\mathrm{t}}\log \frac{{\mathrm{R}}_{\mathrm{o}}}{{\mathrm{R}}_{\mathrm{o}}-\mathrm{x}}\\ \mathrm{k}=\frac{2.303}{30}\log \frac{100{\mathrm{R}}_{\mathrm{o}}}{25{\mathrm{R}}_{\mathrm{o}}}\\ \mathrm{k}=\frac{2.303}{30}\times 0.602\end{array}$

For 93.75% reaction, t is given by 

$\begin{array}{l}\mathrm{t}=\frac{2.303}{\mathrm{k}}\log \frac{\mathrm{a}}{\mathrm{a}-\mathrm{x}}\\ \mathrm{t}=\frac{2.303\times 30}{2.303\times 0.602}\log \frac{100}{100-93.75}\\ \mathrm{t}=\frac{30}{0.602}\log 16=\frac{30\times 1.20}{0.602}=59.8\approx 60\min \end{array}$