$$ \begin{aligned} 28 &= \frac{100\cdot(170-x)}{50}&& \text{multiply ALL terms by } \color{blue}{ 50 }. \\[1 em]50\cdot28 &= 50 \cdot \frac{100\cdot(170-x)}{50}&& \text{cancel out the denominators} \\[1 em]1400 &= -100x+17000&& \text{ swap the left and right side of the equation } \\[1 em]-100x+17000 &= 1400&& \text{ move 17000 to the right } \\[1 em]-100x &= 1400-17000&& \\[1 em]-100x &= -15600&& \text{ divide both sides by $ -100 $ } \\[1 em]x &= \frac{-15600}{-100}&& \\[1 em]x &= 156&& \\[1 em] \end{aligned} $$

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