$$ \begin{aligned} 3x\cdot\frac{40}{100}+x\cdot\frac{17}{100} &= 280000&& \text{multiply ALL terms by } \color{blue}{ 100 }. \\[1 em]1003x\cdot\frac{40}{100}+100x\cdot\frac{17}{100} &= 100\cdot280000&& \text{cancel out the denominators} \\[1 em]120x+17x &= 28000000&& \text{simplify left side} \\[1 em]137x &= 28000000&& \text{ divide both sides by $ 137 $ } \\[1 em]x &= \frac{28000000}{137}&& \\[1 em] \end{aligned} $$

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