Subtract $ \dfrac{5}{y} $ from $ \dfrac{4}{5} $ to get $ \dfrac{ \color{purple}{ 4y-25 } }{ 5y }$.

To subtract raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the first fraction by $ \color{blue}{ y }$ and the second by $\color{blue}{ 5 }$.

$$ \begin{aligned} \frac{4}{5} - \frac{5}{y} & = \frac{ 4 \cdot \color{blue}{ y }}{ 5 \cdot \color{blue}{ y }} - \frac{ 5 \cdot \color{blue}{ 5 }}{ y \cdot \color{blue}{ 5 }} = \\[1ex] &=\frac{ \color{purple}{ 4y } }{ 5y } - \frac{ \color{purple}{ 25 } }{ 5y }=\frac{ \color{purple}{ 4y-25 } }{ 5y } \end{aligned} $$