$$\left. \begin{array} { l } { 5 + 7 z = 10 z + 20 } \end{array} \right.$$
$z=-5$
$$5+7z-10z=20$$
$$5-3z=20$$
$$-3z=20-5$$
$$-3z=15$$
$$z=\frac{15}{-3}$$
$$z=-5$$
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