$$ \begin{aligned} 100-4 \cdot \frac{u}{3} &= 5u+\frac{6}{4}+6&& \text{multiply ALL terms by } \color{blue}{ 12 }. \\[1 em]12\cdot100-12\cdot4 \cdot \frac{u}{3} &= 12\cdot5u+12\cdot\frac{6}{4}+12\cdot6&& \text{cancel out the denominators} \\[1 em]1200-16u &= 60u+18+72&& \text{simplify left and right hand side} \\[1 em]-16u+1200 &= 60u+90&& \text{move the $ \color{blue}{ 60u } $ to the left side and $ \color{blue}{ 1200 }$ to the right} \\[1 em]-16u-60u &= 90-1200&& \text{simplify left and right hand side} \\[1 em]-76u &= -1110&& \text{ divide both sides by $ -76 $ } \\[1 em]u &= \frac{-1110}{-76}&& \\[1 em]u &= \frac{555}{38}&& \\[1 em] \end{aligned} $$

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