$\left. \begin{matrix}    {{N}_{A}}=\frac{{{N}_{{}^\circ A}}}{{{2}^{(\frac{t}{{{T}_{\frac{1}{2}}}})}}}  \\    {{N}_{B}}=\frac{{{N}_{{}^\circ B}}}{{{2}^{(\frac{t}{{{{{T}'}}_{\frac{1}{2}}}})}}}  \\ \end{matrix} \right\}$

$\Rightarrow (Tedad\,\,Zarrat\,\,Vapashi\,\,Shodeh)\,\,{{{N}'}_{A}}={{N}_{{}^\circ A}}(1-\frac{1}{{{2}^{(\frac{t}{{{T}_{\frac{1}{2}}}})}}})\xrightarrow{{{T}_{\frac{1}{2}}}=2{{{{T}'}}_{\frac{1}{2}}},{{{{N}'}}_{A}}=3{{N}_{B}},{{N}_{{}^\circ A}}=\frac{1}{4}{{N}_{{}^\circ B}}}$

$\frac{1}{4}{{N}_{{}^\circ B}}(1-\frac{1}{{{2}^{(\frac{t}{{{T}_{\frac{1}{2}}}})}}})=3\frac{{{N}_{{}^\circ  B}}}{{{2}^{(\frac{2t}{{{T}_{\frac{1}{2}}}})}}}\xrightarrow{{{2}^{(\frac{t}{{{T}_{\frac{1}{2}}}})}}=x}\frac{1}{4}(1-\frac{1}{x})=\frac{3}{{{x}^{2}}}$ 

$12={{x}^{2}}-x\Rightarrow {{x}^{2}}-x-12=0\Rightarrow (x-4)(x+3)=0$ 

$\left\{ \begin{matrix}    x=4\xrightarrow{x={{2}^{(\frac{t}{{{T}_{\frac{1}{2}}}})}}}\frac{t}{{{T}_{\frac{1}{2}}}}=2\Rightarrow t=2{{T}_{\frac{1}{2}}}  \\    x=-3  \\ \end{matrix} \right.\Rightarrow \frac{{{N}_{B}}}{{{N}_{{}^\circ B}}}=\frac{1}{\frac{2t}{{{2}^{{{T}_{\frac{1}{2}}}}}}}=\frac{1}{16}$

$\Rightarrow Darsade\,\,Vapashi\,\,Shodeh=(1-\frac{1}{16})\times 100=93/75$