$$ \begin{aligned} 3 \cdot \frac{x}{4}+5\frac{x}{2} &= 13&& \text{multiply ALL terms by } \color{blue}{ 4 }. \\[1 em]4\cdot3 \cdot \frac{x}{4}+4\cdot5\frac{x}{2} &= 4\cdot13&& \text{cancel out the denominators} \\[1 em]3x+10x &= 52&& \text{simplify left side} \\[1 em]13x &= 52&& \text{ divide both sides by $ 13 $ } \\[1 em]x &= \frac{52}{13}&& \\[1 em]x &= 4&& \\[1 em] \end{aligned} $$

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