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

Belinda remains in one inertial frame with $v=5c/13$, so the time dilation factor is $\sqrt{1-v^2/c^2}=12/13$. Her elapsed proper time isThe reunion event is shared by both twins, making their clock readings directly comparable without any convention for synchronizing separated moving clocks.