Combine like terms

Subtract $ \dfrac{2}{5} $ from $ \dfrac{5}{2} $ to get $ \dfrac{ \color{purple}{ 21 } }{ 10 }$.

To subtract fractions they must have the same denominator.
We can create a common denominator by multiplying the first fraction by $ \color{blue}{ 5 }$ and the second by $\color{blue}{ 2 }$.

$$ \begin{aligned} \frac{5}{2} - \frac{2}{5} & = \frac{ 5 \cdot \color{blue}{ 5 }}{ 2 \cdot \color{blue}{ 5 }} - \frac{ 2 \cdot \color{blue}{ 2 }}{ 5 \cdot \color{blue}{ 2 }} = \\[1ex] &=\frac{ \color{purple}{ 25 } }{ 10 } - \frac{ \color{purple}{ 4 } }{ 10 }=\frac{ \color{purple}{ 21 } }{ 10 } \end{aligned} $$