If a coin rolls such that its complete perimeter touches the other surface exactly once it will move the distance of its perimeter. In this case the total distance is 3x the perimeter so the edge answer is 3. Shape doesn’t matter.
If you’re wondering how many times the traveling coin 360s you have to account for the fact that it is rotating both because it’s rolling and because it’s going around the the other thing. You can simplify this by just imagining it’s the edge again, but the edge is going along the radius of the inner and traveling coin (3+1).
Dumb it down. How far does a point travel? X units. X units / coin perimeter = coin rotations required to travel X. Ergo the only question is whether the distance traveled is along the perimeter of the 3r coin or if it’s based on the center of the traveling coin (3+1).
If you’re wondering how many times the traveling coin 360s you have to account for the fact that it is rotating both because it’s rolling and because it’s going around the the other thing. You can simplify this by just imagining it’s the edge again, but the edge is going along the radius of the inner and traveling coin (3+1).
Dumb it down. How far does a point travel? X units. X units / coin perimeter = coin rotations required to travel X. Ergo the only question is whether the distance traveled is along the perimeter of the 3r coin or if it’s based on the center of the traveling coin (3+1).