Rydberg formula is given by:

where,
= Rydberg constant = 
= wavelength
and
are the level of transitions.
Now, for
= 2 and
= 6

= 
= 
= 
= 

= 
= 
= 
Now, for
= 2 and
= 5

= 
= 
= 

= 
= 
= 
Now, for
= 2 and
= 4

= 
= 
= 

= 
= 
= 
Now, for
= 2 and
= 3

= 
= 
= 

= 
= 
= 
A defining feature of a material that is a good insulator would be its ability to stop or cancel an electric charge of another material. This happens because the molecules in the material are stable and there are no free electrons that would allow the charges to flow.
Answer:
From the image the answer is 24.