Two radioactive substances $A$ and $B$ have same number of initial nuclei. If the half lives of $A$ and $B$ are 1.5 days and 4.5 days respectively, then the ratio of the number of nuclei remaining in $A$ and $B$ after 9 days is

If the difference in the frequencies of the first and second lines of Lyman series of hydrogen atom is $f$, then the difference in frequencies of the first and second lines of Balmer series of hydrogen atom is

The average energy of a neutron produced in the fission of ${ }_{92}^{235} \mathrm{U}$ is

If $96.875 \%$ of a radioactive substance decays in 10 days, then the half life of the substance is (in days)