-30 I think sorry if I’m wrong
Gravitational potential energy :)
Answer:
Approximate linear dimension is 2 light years.
Explanation:
Radius of the spiral galaxy r = 62000 LY
Thickness of the galaxy h = 700 LY
Volume of the galaxy = πr²h
= (3.14)(62000)²(700)
= (3.14)(62)²(7)(10)⁸
= 84568×10⁸
=
(LY)³
Since galaxy contains number of stars = 1078 billion stars ≈ 
Now volume covered by each star of the galaxy = 
= 
= 7.839 Light Years
Now the linear dimension across the volume
= 
= 
= 1.99 LY
≈ 2 Light Years
Therefore, approximate linear dimension is 2 light years.
For a cylinder that has both ends open resonant frequency is given by the following formula:

Where n is the resonance node, v is the speed of sound in air and L is the length of a cylinder.
The fundamental frequency is simply the lowest resonant frequency.
We find it by plugging in n=1:

To find what harmonic has to be excited so that it resonates at f>20Hz we simply plug in f=20 Hz and find our n:

We can see that any resonant frequency is simply a multiple of a base frequency.
Let us find which harmonic resonates with the frequency 20 Hz:

Since n has to be an integer, final answer would be 323.
Different wavelengths of light are seen as colors