What is the order of a radioactive decay reaction?

$A$ radioactive substance is required to have an activity of $0.01 \mu Ci$ in the human body after $24 \ hours$. The half-life of the radioactive substance is $6 \ hours$. What is the maximum initial activity that can be administered (in $\mu Ci$)?

$A$ radioactive sample $(Z = 22)$ decreases by $90\%$ in $10 \ \text{years}$. What will be the half-life of the sample in $\text{years}$?

The half-life of $_{38}^{90}Sr$ is $20 \text{ years}$. If a sample having an initial activity of $8000 \text{ dis/min}$ is taken,what would be its activity after $80 \text{ years}$?

If $87.5\%$ of a given substance disintegrates in $40 \text{ minutes}$,what is the half-life of the radioactive substance?

$87.5\%$ decomposition of a radioactive substance completes in $3$ hours. What is the half-life of that substance?