Hi,
Let us denote first train with T₁, its distance d₁, its speed v₁ = 85 km/h.
Similarly, second train would be with T₂, its distance d₂, its speed v₂ = 80 km/h. Also, let d be total distance, d = 495 km.
Given being the fact that train T₁ travels north, and train T₂ travels south, their path is on a straight vertical line. Please create a simple drawing, you will see the vertical line immediately.
Then, we need to find the time t, when d₁ + d₂ = d = 495 km (1)
Each speed v₁, and v₂ is constant, therefore we apply very well known formula:
d₁ = v₁·t, and d₂ = v₂·t (2).
From (1) and (2) we have:
v₁·t + v₂·t = d, or (v₁ + v₂)t = d ⇒ t = d/(v₁ + v₂) = 495/(80 + 85) = 495/165 = 3 hours.
So, it takes 3 hours to have those 2 trains 495 km apart.
Green eyes.
