How many spectral lines are obtained when an electron in a hydrogen atom undergoes a transition from the $7^{th}$ Bohr orbit to the $2^{nd}$ orbit?

Which of the following relations is applicable for any $H$-like species?

What is the wave number of a photon emitted during the transition from orbit $n=4$ to $n=2$ in a hydrogen atom (in $cm^{-1}$)? $[R_{H}=109677 \ cm^{-1}]$

If the maximum kinetic energy of photoelectrons ejected from a metal surface when it is irradiated with a radiation of frequency $4 \times 10^{14} \ s^{-1}$ is $6.63 \times 10^{-20} \ J$,then the threshold frequency of the metal is

The difference between the radii of $3^{rd}$ and $2^{nd}$ orbit of $H$ atom is $x \text{ pm}$. The difference between the radii of $4^{th}$ and $3^{rd}$ orbit of $Li^{2+}$ ion is $y \text{ pm}$. The ratio $y:x$ is equal to:

In a hydrogen atom,if an electron transitions from an $n$-th orbit to the ground state,and the total number of spectral lines observed is $10$,then the value of $n$ is: