$A$ radioactive substance has a half-life of $60 \text{ minutes}$. During $3 \text{ hours}$,the amount of substance decayed would be: (in $\%$)

The half-life of a radioactive substance is $3.6$ days. How much of $20\, mg$ of this radioactive substance will remain after $36$ days?

The half-life of a radioactive substance is $18 \text{ minutes}$. The time interval between its $20 \%$ decay and $80 \%$ decay in minutes is

$A$ radioactive material has a half-life of $8$ years. The activity of the material will decrease to about $1/8$ of its original value in .......... $years$.

$A$ $16\, g$ sample of a radioactive element is taken from Bombay to Delhi in $2\, hours$ and it is found that $1\, g$ of the element remains (undisintegrated). The half-life of the element is:

The half-life of the isotope $^{11}Na^{24}$ is $15 \text{ hr}$. How much time does it take for $(7/8)$ of a sample of this isotope to decay (in $\text{ hour}$)?