Past exam of the physics 1 paper of the University of Oxford / 2017 resit / 10 / a

Alison reaches the station at $t_1=1/(5/13)=13/5$ years. Her return worldline iswhile Belinda's is $x_B=-5t/13$. Equating them givesThis is the elapsed Earth-frame time, measurable by synchronized clocks in that inertial frame. A signal announcing the reunion reaches a clock physically on Earth later; its light-travel delay is not part of the requested frame-time comparison.