# Homework Help: Ratio Question

1. Jan 24, 2016

### Hannah755

1. The problem statement, all variables and given/known data
The diameter of our disk-shaped galaxy, the Milky Way, is about 1.8E+5 light-years (ly). The distance to the Andromeda galaxy, which is the spiral galaxy nearest to the Milky Way, is about 1.8E+6 ly. If a scale model represents the Milky Way and Andromeda galaxies as dinner plates 26.9 cm in diameter, determine the distance between the centers of the two plates.

2. Relevant equations
1.8E5/1.8E6 = 26.9cm/x

3. The attempt at a solution
Hey! So I attempted this question as a ratio: 1.8E5/1.8E6 = 26.9cm/x, and then solved for x, getting 269cm. I tried inputting this as the answer which was incorrect. Then I noticed that the question was asking for the distance between the centres of the 2 plates, so I tried adding both their radiuses (radii?), and got 269cm + 13.45cm + 13.45cm = 295.9cm. I tried inputting this as the answer, but it was incorrect as well. I feel like I might be completely messing up an incredibly simple question :P Thanks for any help!

2. Jan 24, 2016

### haruspex

You are not told exactly what the distance between the galaxies refers to. the most reasonable guess is that it is centre to centre, so there is no reason to add or subtract any radii. (But if there were, I think you would be subtracting, not adding.)
I see that the intergalactic distance given is significantly understated. It's about 2.5E6ly. This makes me suspect a typo in the problem, repeating the 1.8 instead of stating 2.5.