$$ \begin{aligned} \frac{10}{28} &= \frac{15}{x}&& \text{multiply ALL terms by } \color{blue}{ 28x }. \\[1 em]28x\cdot\frac{10}{28} &= 28x\cdot\frac{15}{x}&& \text{cancel out the denominators} \\[1 em]10x &= 420&& \text{ divide both sides by $ 10 $ } \\[1 em]x &= \frac{420}{10}&& \\[1 em]x &= 42&& \\[1 em] \end{aligned} $$
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