I am doing an extra problem from my textbook as review and I am stumped. An Earth based observer sees to spaceships approaching Earth in the same direction. Spaceship A is travelling at 0.500c and Spaceship B is travelling at 0.800c. How fast is spaceship A traveleing as viewed by an observer on spaceship B? I used a velocity transformation like: (0.500c - 0.800c)/sqrt(1-[(0.5c)(0.8c)/(c^2)]) The book gets -0.500c. I am finding it difficult to see that the Earth based observer and the spaceship moving at 0.800c observe that spaceship going the same speed. Thanks for your help.