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Answer

$$\frac{x^2}{20}-\frac{1}{5}=\frac{x^2-4}{20}$$

Explanation

$$ \begin{aligned}\frac{x^2}{20}-\frac{1}{5}& \xlongequal{ \color{blue}{ \text{\normalsize{ \htmlClass{explanationCircle explanationCircle1}{\textcircled {1}} } }}}\frac{x^2-4}{20}\end{aligned} $$
①

Subtract $ \dfrac{1}{5} $ from $ \dfrac{x^2}{20} $ to get $ \dfrac{ \color{purple}{ x^2-4 } }{ 20 }$.

To subtract raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the second fraction by $\color{blue}{4}$.

$$ \begin{aligned} \frac{x^2}{20} - \frac{1}{5} & = \frac{ x^2 }{ 20 } - \frac{ 1 \cdot \color{blue}{ 4 }}{ 5 \cdot \color{blue}{ 4 }} = \\[1ex] &=\frac{ \color{purple}{ x^2 } }{ 20 } - \frac{ \color{purple}{ 4 } }{ 20 }=\frac{ \color{purple}{ x^2-4 } }{ 20 } \end{aligned} $$
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