$$ \begin{aligned} 25 &= \frac{u}{4}-13&& \text{multiply ALL terms by } \color{blue}{ 4 }. \\[1 em]4\cdot25 &= 4 \cdot \frac{u}{4}-4\cdot13&& \text{cancel out the denominators} \\[1 em]100 &= u-52&& \text{ swap the left and right side of the equation } \\[1 em]u-52 &= 100&& \text{ move -52 to the right } \\[1 em]u &= 100+52&& \\[1 em]u &= 152&& \\[1 em]u &= 152&& \\[1 em] \end{aligned} $$

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