$$ \begin{aligned} \frac{2}{5}\cdot\frac{2}{5}b &= 4\cdot\frac{2}{5}&& \text{multiply ALL terms by } \color{blue}{ 5 }. \\[1 em]5 \cdot \frac{2}{5}\cdot\frac{2}{5}b &= 5\cdot4\cdot\frac{2}{5}&& \text{cancel out the denominators} \\[1 em]\frac{4}{5}b &= 8&& \text{multiply ALL terms by } \color{blue}{ 5 }. \\[1 em]5 \cdot \frac{4}{5}b &= 5\cdot8&& \text{cancel out the denominators} \\[1 em]4b &= 40&& \text{ divide both sides by $ 4 $ } \\[1 em]b &= \frac{40}{4}&& \\[1 em]b &= 10&& \\[1 em] \end{aligned} $$

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