Add $ \dfrac{2}{5} $ and $ \dfrac{4}{7} $ to get $ \dfrac{ \color{purple}{ 34 } }{ 35 }$.

To add fractions they must have the same denominator.
We can create a common denominator by multiplying the first fraction by $ \color{blue}{ 7 }$ and the second by $\color{blue}{ 5 }$.

$$ \begin{aligned} \frac{2}{5} + \frac{4}{7} & = \frac{ 2 \cdot \color{blue}{ 7 }}{ 5 \cdot \color{blue}{ 7 }} + \frac{ 4 \cdot \color{blue}{ 5 }}{ 7 \cdot \color{blue}{ 5 }} = \\[1ex] &=\frac{ \color{purple}{ 14 } }{ 35 } + \frac{ \color{purple}{ 20 } }{ 35 }=\frac{ \color{purple}{ 34 } }{ 35 } \end{aligned} $$