A radioactive substance emits

The half-life of a radioactive substance is $3.6$ days. How much of $20\, mg$ of this radioactive substance will remain after $36$ days ............. $mg$

$A$ and $B$ are two radioactive substances whose half lives are $1$ and $2$ years respectively. Initially $10\, g$ of $A$ and $1\,g$ of $B$ is taken. The time (approximate) after which they will have same quantity remaining is ........... $years$

The half life of a radioactive sample undergoing $\alpha$ - decay is $1.4 \times 10^{17}$ s. If the number of nuclei in the sample is $2.0 \times 10^{21}$, the activity of the sample is nearly

The decay constant of a radio isotope is $\lambda$. If  $A_1$ and $A_2$ are its activities at times $t_1$ and $t_2$  respectively, the number of nuclei which have decayed during the time $(t_1 - t_2)$ 

Radioactive element decays to form a stable nuclide, then the rate of decay of reactant $\left( {\frac{{dN}}{{dt}}} \right)$ will vary with time $(t) $ as shown in figure