The lorentz Transformation eq enables us to transform velocity from one frame of reference to another, in relative motion with respect to it.
Let S and S' be two inertial frames in relative motion so that S; moves with uniform velocity v to the right. alog the X axis Relative to S
Let u and u' be the velocities of a particle measurd in inertial frames S and S'
so u'=u-v/(1-uv/c^2)
as u=v=c so
u'=u-v
u'=0.500-0.400
u'=0.100c