$$ \begin{aligned} 5 \cdot \frac{y}{13}-\frac{21}{52} &= \frac{y}{4}&& \text{multiply ALL terms by } \color{blue}{ 52 }. \\[1 em]52\cdot5 \cdot \frac{y}{13}-52\cdot\frac{21}{52} &= 52 \cdot \frac{y}{4}&& \text{cancel out the denominators} \\[1 em]20y-21 &= 13y&& \text{move the $ \color{blue}{ 13y } $ to the left side and $ \color{blue}{ -21 }$ to the right} \\[1 em]20y-13y &= 21&& \text{simplify left side} \\[1 em]7y &= 21&& \text{ divide both sides by $ 7 $ } \\[1 em]y &= \frac{21}{7}&& \\[1 em]y &= 3&& \\[1 em] \end{aligned} $$

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