The time elapsed between 33 % and 67 % completion of a first order reaction is 30 minutes. What is the time needed for 25 % completion?

The time elapsed between 33% and 67% completion of a first order reaction is 30 minutes

$$\begin{gathered} \mathrm{K}=\ln \left(\frac{0.67}{0.33}\right)know if \\ K=\frac{1}{30}\ln \left(\frac{0.67}{0.33}\right) \\ \Rightarrow {\mathrm{t}}_{1/2}=30 \min \end{gathered}$$

 The time needed for 25% completion,

$$\begin{gathered} \mathrm{K}\times \mathrm{t}=\ln \left(\frac{100}{75}\right) \\ \Rightarrow \mathrm{t}=12.5 \min \end{gathered}$$