Consider the following system of equations:
\[
\begin{cases}
5x + 7y = 8 \\
2x + qy = r
\end{cases}
\]
where \(q\) and \(r\) are constants. If \(q\) and \(r\) are chosen such that this system has infinitely many solutions \((x, y)\), find \(q + r = \underline{~~~~~~~~~~}\). Let's think step by step and output the final answer within \boxed{}.
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