2 trains pass each other. one train is going 70km/h toward the east; the other is going 50km/h toward the west. a passenger on the eastbound train walks toward the back of the tain at 5 km/h. relative to the walking passenger, what is the speed of the westbound train?

Since the two trains are passing each other, relative to the passengers of the eastbound train who are stationary the westbound train is going 120km/h. Since this particular passenger is walking to the back of the train, they are walking toward the westbound train. Thus you subtract the speed of the passenger from 120 to get 115km/h. Hope this helps. Feel free to ask me any questions.

ok, so how did i know that the two trains are passing ea other at 120 km/h...just add the numbers together? Im sorry - it's been 5+ years since ive done this and i have to review for a content exam in three weeks! bleh!

Lol yeah no problem. So because the trains are going in opposite directions you add the speeds together to get the relative speed for the passengers. If the trains are going in the same direction you would subtract the speeds. Just think about it in real life. When you are driving alongside a car going in the same direction its not going very fast compared to you, but if a car passes you in the opposite lane it looks like it blasts past you. Hope that helps