Question

If $N_0$ is the original mass of the substance of half-life period $r_{t/2}=5$ years, then the amount of substance left over 15 years is _____.

Answer 1

No. of half lives $n=\frac{15}{5}=3$

Now, $\frac{N}{N_0}=\frac{1}{2^n}\Rightarrow N=\frac{N_0}{2^n}=\frac{N_0}{8}$