What is the percentage of a radioactive element decayed after $20 \ s$ if its half-life is $4 \ s$?

$8 \, g$ of the radioactive isotope,cesium$-137$ were collected on February $1$ and kept in a sealed tube. On July $1$,it was found that only $0.25 \, g$ of it remained. So the half-life period of the isotope is ....... $days$.

What is the order of radioactive decay of a substance?

The decay constant of a radioactive element is $3 \times 10^{-6} \ min^{-1}$. Its half-life is

During a nuclear explosion,one of the products is $^{90}Sr$ with a half-life of $6.93 \; \text{years}$. If $1 \; \mu g$ of $^{90}Sr$ was absorbed in the bones of a newly born baby in place of $Ca$,how much time,in years,is required to reduce it by $90 \%$ if it is not lost metabolically?

The amount of $_{53}I^{128}$ $(t_{1/2} = 25 \ minutes)$ left after $50 \ minutes$ will be