$$\frac{6}{5} = 1+\frac{r}{100}$$

$$ \begin{aligned} \frac{6}{5} &= 1+\frac{r}{100}&& \text{multiply ALL terms by } \color{blue}{ 100 }. \\[1 em]100\cdot\frac{6}{5} &= 100\cdot1+100 \cdot \frac{r}{100}&& \text{cancel out the denominators} \\[1 em]120 &= 100+r&& \text{ swap the left and right side of the equation } \\[1 em]100+r &= 120&& \text{ move 100 to the right } \\[1 em]r &= 120-100&& \\[1 em]r &= 20&& \\[1 em]r &= 20&& \\[1 em] \end{aligned} $$

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