The disintegration constant for a radioactive substance is $0.58 \, hr^{-1}$. Its half-life period is $........ \, hr$.

The half-life of ${}^{65}Zn$ is $245 \ days$. After $x$ days, $75\%$ of original activity remained. The value of $x$ in days is . . . . . . . (Nearest integer)
(Given $: \log 3=0.4771$ and $\log 2=0.3010$ )

The unit of the decay constant for radioactive disintegration is:

Half-life of a radioactive substance which disintegrates by $75\%$ in $60 \ min$ will be ......... $min$.

If $2.0 \ g$ of a radioactive isotope has a half-life of $20 \ hr$,the half-life of $0.5 \ g$ of the same substance is ........ $hr$.

The half-life of $_6C^{14}$ if its decay constant $k$ or $\lambda$ is $2.31 \times 10^{-4} \ yr^{-1}$ is: