Rules of the Road: BOTH INTERNATIONAL & INLAND You are heading due east (090°) and observe a ve...

This is terrible. Why is it not NW?

It should be northwest. If it was southwest you would observe both sidelights.

So which is the correct answer? NW or SW?

SW (225 deg) is the correct answer. I draw pictures on a maneuvering board to help. But you can think of it this way: If you are meeting another vessel in a red to red, or port to port meeting situation, the other vessel is going in an almost opposite direction. In this case the other vessel would be travelling on a course as close to 270 deg as the answer allows. If the other vessel was travelling NW you would not see red light, only a stern light. Draw it out and remember your Arc of Visibilities. I hope this helps.

The reciprocal of your course is 270. If the other vessel is on your beam on course 315, that is a 45 degree difference. The sideight would only show 22.5 aft of the beam. Anything more than 22.5 degree difference and your looking at a stern light. Only way to see both sidelights would be if the orher vessel was on course 180.