In a mean life of a radioactive sample

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 ............. $mg$

The mean life of a radioactive sample are $30\,year$ and $60\,year$ for $\alpha -$ emission and $\beta -$ emission respectively. If the sample decays both by $\alpha -$ emission and $\beta -$ emission simultaneously, then the time after which, only one-fourth of the sample remain is approximately ............ $years$

Write the definition and unit of decay constant. 

Two radioactive nuclei $A$ and $B$ both convert into a stable nucleus $C$. At time $t = 0$ nuclei of $A$ are $4N_0$ and that of $B$ are $N_0$. Half life of $A$ is $1\, min$ and that of $B$ is $2\, min$. initially number of nuclei of $C$ are zero. At what time rate of disintegrations of $A$ and $B$ are equal .......... $min$

Two radioactive materials $A$ and $B$ have decay constants $25 \lambda$ and $16 \lambda$ respectively. If initially they have the same number of nuclei, then the ratio of the number of nuclei of $B$ to that of $A$ will be "$e$" after a time $\frac{1}{a \lambda}$. The value of $a$ is $......$