Multiply $5$ by $ \dfrac{x}{20} $ to get $ \dfrac{ 5x }{ 20 } $.

Step 1: Write $ 5 $ as a fraction by putting $ \color{red}{1} $ in the denominator.

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} 5 \cdot \frac{x}{20} & \xlongequal{\text{Step 1}} \frac{5}{\color{red}{1}} \cdot \frac{x}{20} \xlongequal{\text{Step 2}} \frac{ 5 \cdot x }{ 1 \cdot 20 } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ 5x }{ 20 } \end{aligned} $$

Multiply $10$ by $ \dfrac{y}{20} $ to get $ \dfrac{ 10y }{ 20 } $.

Step 1: Write $ 10 $ as a fraction by putting $ \color{red}{1} $ in the denominator.

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} 10 \cdot \frac{y}{20} & \xlongequal{\text{Step 1}} \frac{10}{\color{red}{1}} \cdot \frac{y}{20} \xlongequal{\text{Step 2}} \frac{ 10 \cdot y }{ 1 \cdot 20 } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ 10y }{ 20 } \end{aligned} $$

Add $ \dfrac{5x}{20} $ and $ \dfrac{10y}{20} $ to get $ \dfrac{ 5x + 10y }{ \color{blue}{ 20 }}$.

To add expressions with the same denominators, we add the numerators and write the result over the common denominator.

$$ \begin{aligned} \frac{5x}{20} + \frac{10y}{20} & = \frac{5x}{\color{blue}{20}} + \frac{10y}{\color{blue}{20}} =\frac{ 5x + 10y }{ \color{blue}{ 20 }} \end{aligned} $$