$$x\cdot(1+\frac{x}{100}) = 75$$
Answer
$$ \begin{matrix}x_1 = 50 & x_2 = -150 \\[1 em] \end{matrix} $$
Explanation
$$ \begin{aligned} x\cdot(1+\frac{x}{100}) &= 75&& \text{simplify left side} \\[1 em]x \cdot \frac{x+100}{100} &= 75&& \\[1 em]\frac{x^2+100x}{100} &= 75&& \text{multiply ALL terms by } \color{blue}{ 100 }. \\[1 em]100 \cdot \frac{x^2+100x}{100} &= 100\cdot75&& \text{cancel out the denominators} \\[1 em]x^2+100x &= 7500&& \text{move all terms to the left hand side } \\[1 em]x^2+100x-7500 &= 0&& \\[1 em] \end{aligned} $$
$ x^{2}+100x-7500 = 0 $ is a quadratic equation.
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