USAMO 2009 Problem 6

Let $s_1, s_2, s_3, \dots$ be an infinite, non-constant sequence of rational numbers, meaning it is not the case that $s_1 = s_2 = s_3 = \dots$. Suppose that $t_1, t_2, t_3, \dots$ is also an infinite, non-constant sequence of rational numbers with the property that $(s_i - s_j)(t_i - t_j)$ is an integer for all $i$ and $j$. Prove that there exists a rational number $r$ such that $(s_i - s_j)r$ and $(t_i - t_j)/r$ are integers for all $i$ and $j$.