$$\frac{4}{5} = \frac{u-4}{10}$$

$$ \begin{aligned} \frac{4}{5} &= \frac{u-4}{10}&& \text{multiply ALL terms by } \color{blue}{ 10 }. \\[1 em]10\cdot\frac{4}{5} &= 10 \cdot \frac{u-4}{10}&& \text{cancel out the denominators} \\[1 em]8 &= u-4&& \text{ swap the left and right side of the equation } \\[1 em]u-4 &= 8&& \text{ move -4 to the right } \\[1 em]u &= 8+4&& \\[1 em]u &= 12&& \\[1 em]u &= 12&& \\[1 em] \end{aligned} $$

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