[Ether]

Particle "A" is traveling at c/2 in the +X direction in inertial reference frame XY. Particle "B" is traveling at speed c/2 in the -X direction in the same reference frame. See Figure 1.

In a second inertial reference frame X'Y', particle "B" is at rest. How fast is particle "A" traveling in this second reference frame? See Figure 2.