Consider that from the origin O of the inertial reference frame O start at the origin of time a particle that moves with speed u<c and a photon with speed c both in the positive direction of the OX axis. At each point of that axis we find a clock all the clocks being synchronized following the procedure proposed by Einstein. After a given time t of motion the photon arrives at the location of a clock C(x)[x=ct,y=0). What is the reading of a clock located where the particle arrives when the photon arrives at the location mentioned above? Thanks.