$ N = \frac{N_0}{2^n}$

where $ n = \frac{t}{t_\frac{1}{2}}$

$N = \frac{N_0}{16} = \frac{N_0}{2^4}$

$\Rightarrow n = \frac{t}{t_\frac{1}{2}} = 4$

$t = 4 t_\frac{1}{2}= 400\mu s$