\(\therefore\) \(\frac{1}{8} = {\left( {\frac{1}{2}} \right)^{\frac{{15}}{{{\tau _{1/2}}}}}} \Rightarrow {\left( {\frac{1}{2}} \right)^3} = {\left( {\frac{1}{2}} \right)^{\frac{{15}}{{{\tau _{1/2}}}}}}\)
\(\therefore\) \(\frac{{15}}{{{\tau _{\frac{1}{2}}}}} = 3\,\, \Rightarrow \,\,\,{\tau _{\frac{1}{2}}} = 5\,\min \)
$ _{90}{X^{200}}{ \to _{80}}{Y^{168}} $
(જ્યાં $\lambda$ ક્ષય નિયાતાંક છે)