Add $1$ and $ \dfrac{3}{5x-20} $ to get $ \dfrac{ \color{purple}{ 5x-17 } }{ 5x-20 }$.

Step 1: Write $ 1 $ as a fraction by putting $ \color{red}{ 1 } $ in the denominator.

Step 2: To add raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the first fraction by $\color{blue}{ 5x-20 }$.

$$ \begin{aligned} 1+ \frac{3}{5x-20} & \xlongequal{\text{Step 1}} \frac{1}{\color{red}{1}} + \frac{3}{5x-20} = \frac{ 1 \cdot \color{blue}{ \left( 5x-20 \right) }}{ 1 \cdot \color{blue}{ \left( 5x-20 \right) }} + \frac{ 3 }{ 5x-20 } = \\[1ex] & \xlongequal{\text{Step 2}} \frac{ \color{purple}{ 5x-20 } }{ 5x-20 } + \frac{ \color{purple}{ 3 } }{ 5x-20 }=\frac{ \color{purple}{ 5x-17 } }{ 5x-20 } \end{aligned} $$