$$d\cdot30 = 3250-\frac{d}{2}\cdot150$$

$$ \begin{aligned} d\cdot30 &= 3250-\frac{d}{2}\cdot150&& \text{multiply ALL terms by } \color{blue}{ 2 }. \\[1 em]2d\cdot30 &= 2\cdot3250-2 \cdot \frac{d}{2}\cdot150&& \text{cancel out the denominators} \\[1 em]60d &= 6500-150d&& \text{move the $ \color{blue}{ -150d } $ to the left side} \\[1 em]60d+150d &= 6500&& \text{simplify left side} \\[1 em]210d &= 6500&& \text{ divide both sides by $ 210 $ } \\[1 em]d &= \frac{6500}{210}&& \\[1 em]d &= \frac{650}{21}&& \\[1 em] \end{aligned} $$

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