Question

Two galaxies on opposite ends of the universe are moving away from the Earth. Each has a velocity of 200,000 km/s relative to the Earth. How fast would an observer in one of those galaxies see the other galaxy moving away? (1 point)
Responses

200,000 km/s
200,000 kilo meter per second

between 300,000 and 400,000 km/s
between 300,000 and 400,000 kilo meter per second

between 200,000 and 300,000 km/s
between 200,000 and 300,000 kilo meter per second

400,000 km/s

Answer 1

Use relativistic velocity addition. For opposite directions,
u = 2v / (1 + v^2/c^2).
With v = 200,000 km/s and c ≈ 300,000 km/s,
u ≈ 400,000 / 1.4444 ≈ 277,000 km/s.

So: between 200,000 and 300,000 km/s.