$$ \begin{aligned} x+\frac{x}{4} &= 125&& \text{multiply ALL terms by } \color{blue}{ 4 }. \\[1 em]4x+4 \cdot \frac{x}{4} &= 4\cdot125&& \text{cancel out the denominators} \\[1 em]4x+x &= 500&& \text{simplify left side} \\[1 em]5x &= 500&& \text{ divide both sides by $ 5 $ } \\[1 em]x &= \frac{500}{5}&& \\[1 em]x &= 100&& \\[1 em] \end{aligned} $$

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