$$ \begin{aligned} d &= 100-\frac{d}{3}&& \text{multiply ALL terms by } \color{blue}{ 3 }. \\[1 em]3d &= 3\cdot100-3 \cdot \frac{d}{3}&& \text{cancel out the denominators} \\[1 em]3d &= 300-d&& \text{move the $ \color{blue}{ -d } $ to the left side} \\[1 em]3d+d &= 300&& \text{simplify left side} \\[1 em]4d &= 300&& \text{ divide both sides by $ 4 $ } \\[1 em]d &= \frac{300}{4}&& \\[1 em]d &= 75&& \\[1 em] \end{aligned} $$

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