$$ \begin{aligned} 20 &= 5-\frac{3}{8}x&& \text{multiply ALL terms by } \color{blue}{ 8 }. \\[1 em]8\cdot20 &= 8\cdot5-8 \cdot \frac{3}{8}x&& \text{cancel out the denominators} \\[1 em]160 &= 40-3x&& \text{ swap the left and right side of the equation } \\[1 em]40-3x &= 160&& \text{ move 40 to the right } \\[1 em]-3x &= 160-40&& \\[1 em]-3x &= 120&& \text{ divide both sides by $ -3 $ } \\[1 em]x &= -\frac{120}{3}&& \\[1 em]x &= -40&& \\[1 em] \end{aligned} $$

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