If $N_0$ is the original mass of the substance of half life $t_{\frac{1}{2}}=4$ years, then the amount of substance left after 12 years is:

$N_0 / 16$ $N_0 / 4$ $N_0 / 8$ $N_0 / 2$

$N_0 / 8$

The correct answer is Option (3) → $N_0 / 8$