$$ \begin{aligned} 40 &= \frac{4}{3}x&& \text{multiply ALL terms by } \color{blue}{ 3 }. \\[1 em]3\cdot40 &= 3 \cdot \frac{4}{3}x&& \text{cancel out the denominators} \\[1 em]120 &= 4x&& \text{ swap the left and right side of the equation } \\[1 em]4x &= 120&& \text{ divide both sides by $ 4 $ } \\[1 em]x &= \frac{120}{4}&& \\[1 em]x &= 30&& \\[1 em] \end{aligned} $$

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