If $\mathrm{N}_0$ is the original mass of the substance of half-life period $\mathrm{t}_{1/2} = 5$ years, then the amount of substance left after 15 years is\\
(1) $\mathrm{N}_0/8$\\
(2) $\mathrm{N}_0/16$\\
(3) $\mathrm{N}_0/2$\\
(4) $\mathrm{N}_0/4$