A radioactive material has a half life of $10$ days. What fraction of the material would remain after $30$ days

In saloons, there is always a characteristics smell due to the ammonia-based chemicals used in hair dyes and other products. Assume the initial concentration of ammonia molecules to be $1000 \,molecules/ m ^3$. Due to air ventilation, the number of molecules leaving in one minute is one tenth of the molecules present at the start of that minute. How long will it take for the concentration of ammonia molecules to reach $1 \,molecule / m ^3$ ?

If $10\%$ of a radioactive material decays in $5$ days, then the amount of original material left after $20$ days is approximately ...............$\%$

The half life period of a radioactive element $X$ is same as the mean life time of another radioactive element $Y$. Initially both of them have the same number of atoms. Then

A radioactive sample has half-life of $5$ years. Probability of decay in $10$ years will be  ........$\%$

A radioactive material decays by simultaneous emission of two particles with respective half lives $1620$ and $810$ years. The time (in years) after which one- fourth of the material remains is