$$ \begin{aligned} \frac{72}{n} &= 9&& \text{multiply ALL terms by } \color{blue}{ n }. \\[1 em]n\cdot\frac{72}{n} &= n\cdot9&& \text{cancel out the denominators} \\[1 em]72 &= 9n&& \text{ swap the left and right side of the equation } \\[1 em]9n &= 72&& \text{ divide both sides by $ 9 $ } \\[1 em]n &= \frac{72}{9}&& \\[1 em]n &= 8&& \\[1 em] \end{aligned} $$

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