1. The problem statement, all variables and given/known data The speed of a particle in Earth's frame is 0.4 c. A second particle goes away from the first one with speed 0.6 c. What is the speed of the second particle in Earth's frame? 2. Relevant equations u=(u'+v)/(1+u'v/c^2) 3. The attempt at a solution I think v=0.4 c and u'=-0.6, but as I use the above equation the solution is not 0.8 c(as it is in my book). If I put v=-0.6 and u=0.4 then the solution is 0.8 c. Which one is correct?