Suppose we wanted to make a scale model or the Local Group of Galaxies, such that the Milky Way Galaxy were

...the size of a marble, that is, about 1 centimeter.

a.) How for away from the Milky Way Galaxy would the Andromeda Galaxy be on the scale?

b.) How far would it be on this scale from the Milky Way Galaxy to the most distant galaxies in the observable universe?

yes, this is my homework. No, I don't know how to do it.

you're help is greatly appreciated.

MW galaxy is 100K ly across, your 1 cm marble.
Andromeda galaxy, M31, is about the same size, another 1 cm marble.
The distance between them now is estimated to be about 2.5M ly or about 25 marbles or 0.25m distant.
The most distant galaxies we can image in the Hubble Deep Field shots are about 12 B ly or about 4800 times farther than the Andromeda Galaxy. That would be 120,000 marbles or 1200 meters or just over 1 kilometer.

well the milky way is about 100000 light years across, so if you say 100000 ly = 1 cm you now have your scale.

Now u just need to find the distance to the galaxies and you have your answer.
(you didnt want me to do it all for you did you?)

a) 20 cm
b) 130,000 cm
Figures are just from the top of my head.