$A$ 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 $ 20 \text{ minutes} $. The time taken between $ 50\% $ decay and $ 87.5\% $ decay of the substance will be

Calculate the time (in $minutes$) interval between $33\%$ decay and $67\%$ decay if the half-life of a substance is $20\, minutes$.

$A$ sample originally contained $10^{20}$ radioactive atoms,which emit $\alpha$-particles. The ratio of $\alpha$-particles emitted in the third year to that emitted during the second year is $0.3$. How many $\alpha$-particles were emitted in the first year?

One mole of radium has an activity of $\frac{1}{3.7} \text{ kilo curie}$. Its decay constant is (Avogadro number $= 6 \times 10^{23} \text{ mol}^{-1}$)

The number of atoms of a radioactive substance of half-life $T$ is $N_{0}$ at $t=0$. The time necessary to decay from $N_{0} / 2$ atoms to $N_{0} / 10$ atoms will be