$$ \begin{aligned} \frac{2}{3}+\frac{3}{10}c &= \frac{2}{3}&& \text{multiply ALL terms by } \color{blue}{ 30 }. \\[1 em]30\cdot\frac{2}{3}+30\frac{3}{10}c &= 30\cdot\frac{2}{3}&& \text{cancel out the denominators} \\[1 em]20+9c &= 20&& \text{ move 20 to the right } \\[1 em]9c &= 20-20&& \\[1 em]9c &= 0&& \text{ divide both sides by $ 9 $ } \\[1 em]c &= \frac{0}{9}&& \\[1 em]c &= 0&& \\[1 em] \end{aligned} $$

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