If the time taken for a radioactive substance to decay $8 \%$ to $77 \%$ is 12 minutes, then the half life of the substance in minutes is

For an observer on the Earth, if a spectral line of wavelength $6600\mathop {\rm{A}}\limits^{\rm{o}}$ emitted by a star is found to be red shifted by $22 \mathop {\rm{A}}\limits^{\rm{o}}$, then the star is

The difference between the frequencies of the first and second Lyman lines of hydrogen atom is ( $R=$ Rydberg constant and $c=$ speed of light in vacuum)

If the half-life of a radioactive element is 12.5 hours, then the time taken to disintegrate 256 g of the substance into 1 g is (in hours)