Add $ \dfrac{3}{4x} $ and $ \dfrac{1}{20x} $ to get $ \dfrac{ \color{purple}{ 64x } }{ 80x^2 }$.

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}{ 20x }$ and the second by $\color{blue}{ 4x }$.

$$ \begin{aligned} \frac{3}{4x} + \frac{1}{20x} & = \frac{ 3 \cdot \color{blue}{ 20x }}{ 4x \cdot \color{blue}{ 20x }} + \frac{ 1 \cdot \color{blue}{ 4x }}{ 20x \cdot \color{blue}{ 4x }} = \\[1ex] &=\frac{ \color{purple}{ 60x } }{ 80x^2 } + \frac{ \color{purple}{ 4x } }{ 80x^2 }=\frac{ \color{purple}{ 64x } }{ 80x^2 } \end{aligned} $$