Solution:
Solution
$$\displaystyle -\frac{dN}{dt}=\lambda N_0$$
where $$\lambda$$(disintegration constant) $$\displaystyle = \frac{0.693}{T_{50}}$$ and $$N_0$$ the number of atoms present initially
$$\displaystyle N_0=\frac{weight}{atomic\quad weight}\times Avogadro's\quad number$$
$$\displaystyle -\frac{dN}{dt}=\frac{0.693}{21\times 60}\times \frac{1}{11}\times 6.02\times 10^{23}$$ dps
$$=3\times 10^{19}$$ dps
