When I went today after the bridge over the river, I thought... It's about how I see the situation. I noticed that just below me (under the bridge) was situated bow boat, sailing directly under the bridge. For how long, the boat will be on second side? How will the solution be different if I go on evenly, from a case if I stood on the spot?How to express numerically? It will be something very complicated geometry, or what will? Thanks very much... And sorry for my bad English.