Half-life of a radioactive element depends upon:

$A$ radioactive sample has an activity $A$ in air. If the sample is kept inside water,then its activity $A^{\prime}$

If the time taken for a radioactive substance to decay from $88 \%$ to $77 \%$ is $12 \text{ minutes}$,then the half-life of the substance in minutes is:

The activities of three radioactive substances $A, B$ and $C$ are represented by the curves $A, B$ and $C$ in the figure. Then their half-lives $T_{\frac{1}{2}}(A) : T_{\frac{1}{2}}(B) : T_{\frac{1}{2}}(C)$ are in the ratio:

$A$ radioactive substance in a laboratory has a half-life of $2 \text{ hours}$. It is considered safe for laboratory use when its activity reduces to $1/64$ of its initial value. After how many hours will the laboratory be safe?

$A$ $280\, \text{day}$ old radioactive substance shows an activity of $6000\, \text{dps}$. $140\, \text{days}$ later, its activity becomes $3000\, \text{dps}$. What was its initial activity? ......... $\text{dps}$