Solve the following math problem step by step. The last line of your response should be of the form Answer: \boxed{$Answer} where $Answer is the answer to the problem.

<image>In $\triangle PQR$, $\overline{ST} \| \overline{R Q}$ . If  $PT=7.5$, $TQ=3$, and $SR=2.5$, find $PS$.