$$ \begin{aligned} 4x-1980 &= (7920-6x)\cdot\frac{25}{12}\cdot\frac{1}{100}&& \text{multiply ALL terms by } \color{blue}{ 300 }. \\[1 em]300\cdot4x-300\cdot1980 &= 300(7920-6x)\cdot\frac{25}{12}\cdot\frac{1}{100}&& \text{cancel out the denominators} \\[1 em]1200x-594000 &= \frac{25\cdot(7920-6x)}{4}&& \text{multiply ALL terms by } \color{blue}{ 4 }. \\[1 em]4\cdot1200x-4\cdot594000 &= 4 \cdot \frac{25\cdot(7920-6x)}{4}&& \text{cancel out the denominators} \\[1 em]4800x-2376000 &= -150x+198000&& \text{move the $ \color{blue}{ -150x } $ to the left side and $ \color{blue}{ -2376000 }$ to the right} \\[1 em]4800x+150x &= 198000+2376000&& \text{simplify left and right hand side} \\[1 em]4950x &= 2574000&& \text{ divide both sides by $ 4950 $ } \\[1 em]x &= \frac{2574000}{4950}&& \\[1 em]x &= 520&& \\[1 em] \end{aligned} $$

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