The half-life of a radioactive element is 100 minutes. The time interval between the stages to 50% and 87.5% decay will be: 

$\mathrm{\Delta T}=\left|{\mathrm{t}}_1-3{\mathrm{t}}_{1/2}\right|=(100-300\times 100)=200\min$

Alternatively:

$\begin{array}{l}\frac{{\mathrm{t}}_{87.5\mathrm{\%}}}{{\mathrm{t}}_{50\mathrm{\%}}}=\frac{\log \frac{100}{(10\mathrm{a}0-87.5)}}{\log \left(\frac{100}{100-50}\right)}\\ {\mathrm{t}}_{87.5\mathrm{\%}}=\frac{{\mathrm{t}}_{50\mathrm{\%}}\times \log \frac{100}{12.5}}{\log 2}\\ =\frac{100\times (\log 100-\log 125+\log 10)}{0.3}\\ =\frac{100\times (2-3\log 5+1)}{0.3}\\ =100\times (3-3\times 0.7)/0.3\end{array}$

$$\begin{gathered} =\frac{100\times 0.9}{0.3}=300\min \\ \left({\mathrm{t}}_{87.5\mathrm{\%}}={\mathrm{t}}_{1/2}\right)\equiv 300-100=200\min \end{gathered}$$