\(\frac{\ln 2}{\left( t _{1 / 2}\right)_{ eq }}=\frac{\ln 2}{\left( t _{1 / 2}\right)_{1}}+\frac{\ln 2}{\left( t _{1 / 2}\right)_{2}}\)
\(\left( t _{1 / 2}\right)_{ eq }=\frac{\left( t _{1 / 2}\right)_{1} \times\left( t _{1 / 2}\right)_{2}}{\left( t _{1 / 2}\right)_{1}+\left( t _{1 / 2}\right)_{2}}\)
\(=\frac{3 \times 4.5}{3+4.5}=\frac{3 \times 4.5}{7.5}=\frac{3 \times 3}{5}=1.8 \; hr\)
$ _{90}{X^{200}}{ \to _{80}}{Y^{168}} $