The distance between points $ A $ and $ B $ is :
$$ d(A, B) = 1 $$To find distance between points $ A(x_1,y_1)$ and $ B(x_2,y_2)$, we use formula:
$$ \color{blue}{ d(A,B) = \sqrt{(x_2 - x_1)^2+(y_2 - y_1)^2} } $$In this example we have:
$$ \begin{aligned} & A \left(\dfrac{ 2 }{ 5 },~\dfrac{ 3 }{ 5 }\right) \implies x_1 = \frac{ 2 }{ 5 } ~~\text{and}~~ y_1 = \frac{ 3 }{ 5 } \\[1 em] & B \left(1,~\dfrac{ 7 }{ 5 }\right) \implies x_2 = 1 ~~\text{and}~~ y_2 = \frac{ 7 }{ 5 } \end{aligned} $$Substituting $ x_1 $, $ x_2 $, $ y_1 $ and $ y_2 $ into the formula above yields:
$$ \begin{aligned} d(A,B) & = \sqrt{\left( 1 - \frac{ 2 }{ 5 } \right)^2 + \left( \frac{ 7 }{ 5 } - \frac{ 3 }{ 5 } \right)^2} \\[1 em] d(A,B) & = \sqrt{ \left(\frac{ 3 }{ 5 }\right)^2 + \left(\frac{ 4 }{ 5 }\right)^2 } \\[1 em] d(A,B) & = \sqrt{ \frac{ 9 }{ 25 } + \frac{ 16 }{ 25 } } \\[1 em] d(A,B) & = \sqrt{ 1 } \\[1 em] d(A,B) & = 1 \end{aligned} $$