$$ \begin{aligned} \frac{2}{3}b &= 20-b&& \text{multiply ALL terms by } \color{blue}{ 3 }. \\[1 em]3 \cdot \frac{2}{3}b &= 3\cdot20-3b&& \text{cancel out the denominators} \\[1 em]2b &= 60-3b&& \text{move the $ \color{blue}{ -3b } $ to the left side} \\[1 em]2b+3b &= 60&& \text{simplify left side} \\[1 em]5b &= 60&& \text{ divide both sides by $ 5 $ } \\[1 em]b &= \frac{60}{5}&& \\[1 em]b &= 12&& \\[1 em] \end{aligned} $$

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