$$ \begin{aligned} d-\frac{1}{3}d &= 20&& \text{multiply ALL terms by } \color{blue}{ 3 }. \\[1 em]3d-3 \cdot \frac{1}{3}d &= 3\cdot20&& \text{cancel out the denominators} \\[1 em]3d-d &= 60&& \text{simplify left side} \\[1 em]2d &= 60&& \text{ divide both sides by $ 2 $ } \\[1 em]d &= \frac{60}{2}&& \\[1 em]d &= 30&& \\[1 em] \end{aligned} $$

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