$$43 = 20+\frac{z}{3}$$

$$ \begin{aligned} 43 &= 20+\frac{z}{3}&& \text{multiply ALL terms by } \color{blue}{ 3 }. \\[1 em]3\cdot43 &= 3\cdot20+3 \cdot \frac{z}{3}&& \text{cancel out the denominators} \\[1 em]129 &= 60+z&& \text{ swap the left and right side of the equation } \\[1 em]60+z &= 129&& \text{ move 60 to the right } \\[1 em]z &= 129-60&& \\[1 em]z &= 69&& \\[1 em]z &= 69&& \\[1 em] \end{aligned} $$

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