\(\Rightarrow \frac{{{R_2}}}{{{R_1}}} = \frac{{{N_2}}}{{{N_1}}}\)

But \(\frac{{{N_2}}}{{{N_1}}} = {\left( {\frac{1}{2}} \right)^{{t_{1/2}}}}\)

\(\Rightarrow \frac{{25}}{{200}} = \frac{1}{8} = {\left( {\frac{1}{2}} \right)^3}\)

\(\Rightarrow \frac{t}{{{t_{1/2}}}} = 3\)

\(\therefore {t_{1/2}} = \frac{t}{3} = \frac{3}{3} = 1\;hour = 60\,minutes\)