$$ \begin{aligned} \frac{1}{5}x-x &= 320&& \text{multiply ALL terms by } \color{blue}{ 5 }. \\[1 em]5 \cdot \frac{1}{5}x-5x &= 5\cdot320&& \text{cancel out the denominators} \\[1 em]x-5x &= 1600&& \text{simplify left side} \\[1 em]-4x &= 1600&& \text{ divide both sides by $ -4 $ } \\[1 em]x &= -\frac{1600}{4}&& \\[1 em]x &= -400&& \\[1 em] \end{aligned} $$

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