Two similar posters have a scale factor of 4:5. The large posters perimeter is 85 inches. Find the small posters perimeter.

If the ratio of the posters' perimeter is 4:5 and the larger one is 85 inches, then we may want to know how much is one "part" of the ratio - so we divide 85 by 5:

85 / 5 = 17

So the larger poster is 5 * 17 = 85, and the smaller one is 4 * 17 = 68.