Answer:
As it is given that
[tex]activity=\lambda= 100\\[/tex]
[tex]Number~of ~counts=100/second\\[/tex]
Using formula
[tex]ln(\frac{N}{N_o})=-\lambda\cdot t\\\\[/tex]
or
[tex]-\frac{ln(\frac{N}{N_o})}{\lambda}= t\\\\[/tex]
where [tex]N=\frac{N_o}{3}[/tex]
[tex]-\frac{ln(\frac{1}{3})}{100}= t\\\\\-\frac{-1.0986}{100}=t\\\\[/tex]
[tex]\frac{1.0986}{100}=t\\1.0986\cdot 10^{-2}seconds=t\\[/tex]