$$ \begin{aligned} \frac{12}{x} &= \frac{48}{72}&& \text{multiply ALL terms by } \color{blue}{ x\cdot72 }. \\[1 em]x\cdot72\cdot\frac{12}{x} &= x\cdot72\cdot\frac{48}{72}&& \text{cancel out the denominators} \\[1 em]864 &= 48x&& \text{ swap the left and right side of the equation } \\[1 em]48x &= 864&& \text{ divide both sides by $ 48 $ } \\[1 em]x &= \frac{864}{48}&& \\[1 em]x &= 18&& \\[1 em] \end{aligned} $$

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