Relativity problem-tough!

Question

Spaceships A and B are moving in the same direction along the same straight line. Spaceship A is moving at 0.85c relative to a stationary observer, and spaceship B is moving at 0.85c relative to spaceship A. What does the stationary observer find for speed of spaceship B?

Answer 1

That would be easy:

0.85c + 0.85c = 1.7c

Answer 2

Don't forget that time passes at half its normal rate at .87c.

Answer 3

Reading Kot Matroskins answer made me die a little inside.

Mullah Abdullah is correct, though I would work it out like so:

s = (v + u) / (1 + (v/c)(u/c))

s = (0.85 + 0.85) / (1 + 0.85*0.85)

s = 1.7 / (1.7225)

s = 0.9869

Answer 4

That would be easy:

tanh (area tanh(0.85) + area tanh(0.85)) c =

tanh (2 area tanh(0.85)) c =

0.98693759071117561683599419448476 c

(actually that would equal 680/689 c)