Add $ \dfrac{1}{5x^3} $ and $ \dfrac{x}{25} $ to get $ \dfrac{ \color{purple}{ 5x^4+25 } }{ 125x^3 }$.

To add raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the first fraction by $ \color{blue}{ 25 }$ and the second by $\color{blue}{ 5x^3 }$.

$$ \begin{aligned} \frac{1}{5x^3} + \frac{x}{25} & = \frac{ 1 \cdot \color{blue}{ 25 }}{ 5x^3 \cdot \color{blue}{ 25 }} + \frac{ x \cdot \color{blue}{ 5x^3 }}{ 25 \cdot \color{blue}{ 5x^3 }} = \\[1ex] &=\frac{ \color{purple}{ 25 } }{ 125x^3 } + \frac{ \color{purple}{ 5x^4 } }{ 125x^3 }=\frac{ \color{purple}{ 5x^4+25 } }{ 125x^3 } \end{aligned} $$