Solution
= Solution
The amplitude at $y$ is proportional to $e^{2\pi ix_0y/8}\sum_{j=0}^{A-1}e^{2\pi ijry/8}$. The geometric sum vanishes unless $ry\equiv0\pmod8$, i.e. unless $y$ is a multiple of $A=8/r$. At those $r$ outcomes its <modulus> is $1/\sqrt r$, so each probability is $1/r$. The phase contains $x_0$, but measurement probabilities do not.