Suppose, A is moving with constant speed c/2 and B is stationary. A fires a beam in direction of his travel. A measures the speed of light as c. B also measures the speed of light as c obviously. But when we ask B to claim what is speed measured by A, then what should be B's answer? I guess the B's answer is according to STR: = (c - c/2)/(1 - (c*c/2)/(c^2)) = c If I am right in above then how can B claim that A measure speed of light as c? Summary: A measures c. B measures c. But how can B claim that A measures c?