The number of half-lives elapsed before $93.75 \%$ of a radioactive sample has decayed is

Half lives of two radioactive substances $A$ and $B$ are respectively $20$ minutes and $40$ minutes. Initially,the sample of $A$ and $B$ have equal number of nuclei. After $80$ minutes,the ratio of the remaining numbers of $A$ and $B$ nuclei is

$A$ mixture consists of two radioactive materials $A_1$ and $A_2$ with half-lives of $20 \ s$ and $10 \ s$ respectively. Initially,the mixture has $40 \ g$ of $A_1$ and $160 \ g$ of $A_2$. The amount of the two in the mixture will become equal after: (in $s$)

$A$ radioactive substance of mass $10 \, g$ is kept in an open container. What is the approximate mass of the substance remaining in the container after two mean lives (in $, g$)?

$A$ radio isotope has a half-life of $75\, \text{years}$. The fraction of the atoms of this material that would decay in $150\, \text{years}$ will be...........$\%$

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