$$\frac{4}{b} = \frac{8}{4}$$

$$ \begin{aligned} \frac{4}{b} &= \frac{8}{4}&& \text{multiply ALL terms by } \color{blue}{ b\cdot4 }. \\[1 em]b\cdot4\cdot\frac{4}{b} &= b\cdot4\cdot\frac{8}{4}&& \text{cancel out the denominators} \\[1 em]16 &= 8b&& \text{ swap the left and right side of the equation } \\[1 em]8b &= 16&& \text{ divide both sides by $ 8 $ } \\[1 em]b &= \frac{16}{8}&& \\[1 em]b &= 2&& \\[1 em] \end{aligned} $$

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