$$ \begin{aligned} & \mathrm{t}_{1 / 2} \propto\left(\mathrm{P}_{\mathrm{o}}\right)^{1-\mathrm{n}} \\\\ & \frac{\left(\mathrm{t}_{1 / 2}\right)_1}{\left(\mathrm{t}_{1 / 2}\right)_2}=\frac{\left(\mathrm{P}_o\right)_1^{1-\mathrm{n}}}{\left(\mathrm{P}_{o_2}\right)_2^{1-\mathrm{n}}} \\\\ & \Rightarrow\left(\frac{4}{2}\right)=\left(\frac{50}{100}\right)^{1-\mathrm{n}} \\\\ & \Rightarrow 2=\left(\frac{1}{2}\right)^{1-\mathrm{n}} \\\\ & \Rightarrow 2=(2)^{\mathrm{n}-1} \\\\ & \Rightarrow \mathrm{n}-1=1 \\\\ & \Rightarrow \mathrm{n}=2 \end{aligned} $$