${\text{N}} = {{\text{N}}_{\text{0}}}{\left( {\frac{1}{2}} \right)^{t/{\tau _{1/2}}}}\, \Rightarrow \,{N_A} = 10{\left( {\frac{1}{2}} \right)^{t/1}}$  અને

${{\text{N}}_{\text{B}}} = 1{\left( {\frac{1}{2}} \right)^{t/2}}$

${N_A} = {N_B} \Rightarrow \,10{\left( {\frac{1}{2}} \right)^t} = {\left( {\frac{1}{2}} \right)^{t/2}}\,\,$   આપેલ છે 

$ \Rightarrow \,\,{\text{10}} = {\left( {\frac{{\text{1}}}{{\text{2}}}} \right)^{ – t/2}}\, \Rightarrow \,\,10 = {2^{t/2}}$

બંને બાજુ $log$ લેતાં,

${\log _{10}}10 = \frac{t}{2}{\log _{10}}2\,$

$\Rightarrow \,1 = \frac{t}{2} \times 0.3010\,$

$\Rightarrow \,t = 6.62\,year$