Multiply $20$ by $ \dfrac{x^3}{5} $ to get $ \dfrac{ 20x^3 }{ 5 } $.

Step 1: Write $ 20 $ as a fraction by putting $ \color{red}{1} $ in the denominator.

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

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

Multiply $ \dfrac{20x^3}{5} $ by $ x $ to get $ \dfrac{ 20x^4 }{ 5 } $.

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

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} \frac{20x^3}{5} \cdot x & \xlongequal{\text{Step 1}} \frac{20x^3}{5} \cdot \frac{x}{\color{red}{1}} \xlongequal{\text{Step 2}} \frac{ 20x^3 \cdot x }{ 5 \cdot 1 } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ 20x^4 }{ 5 } \end{aligned} $$

Add $ \dfrac{20x^4}{5} $ and $ 10 $ to get $ \dfrac{ \color{purple}{ 20x^4+50 } }{ 5 }$.

Step 1: Write $ 10 $ 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 second fraction by $\color{blue}{5}$.

$$ \begin{aligned} \frac{20x^4}{5} +10 & \xlongequal{\text{Step 1}} \frac{20x^4}{5} + \frac{10}{\color{red}{1}} = \frac{ 20x^4 }{ 5 } + \frac{ 10 \cdot \color{blue}{ 5 }}{ 1 \cdot \color{blue}{ 5 }} = \\[1ex] & \xlongequal{\text{Step 2}} \frac{ \color{purple}{ 20x^4 } }{ 5 } + \frac{ \color{purple}{ 50 } }{ 5 }=\frac{ \color{purple}{ 20x^4+50 } }{ 5 } \end{aligned} $$