$$ \begin{aligned} \frac{9}{10} &= \frac{x}{16}&& \text{multiply ALL terms by } \color{blue}{ 80 }. \\[1 em]80\cdot\frac{9}{10} &= 80 \cdot \frac{x}{16}&& \text{cancel out the denominators} \\[1 em]72 &= 5x&& \text{ swap the left and right side of the equation } \\[1 em]5x &= 72&& \text{ divide both sides by $ 5 $ } \\[1 em]x &= \frac{72}{5}&& \\[1 em] \end{aligned} $$

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