$$ \begin{aligned} 4(x+5) &= 4x+5x&& \text{simplify left and right hand side} \\[1 em]4x+20 &= 9x&& \text{move the $ \color{blue}{ 9x } $ to the left side and $ \color{blue}{ 20 }$ to the right} \\[1 em]4x-9x &= -20&& \text{simplify left side} \\[1 em]-5x &= -20&& \text{ divide both sides by $ -5 $ } \\[1 em]x &= \frac{-20}{-5}&& \\[1 em]x &= 4&& \\[1 em] \end{aligned} $$

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