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Lily is in a spaceship moving at speed $V=0.5c$ with respct to a viewer at rest on earth. Her friends james and severus are also in their own spaceships.Lily is watching that Severus is going forward at a speed $V_1=0.8c$ and James is going backward at a speed $V_2=0.85c$.
How fast is James going according to Severus?

\[v_{JS}=\dfrac{0.85c+0.8c}{1+\dfrac {0.85c\times 0.8c}{c^2}}=0.98c\]