#### Answer

$\dfrac{\sqrt{10}}{5}$

#### Work Step by Step

Rationalizing the denominator of the given expression, $
\sqrt{\dfrac{2}{5}}
,$ we find:
\begin{array}{l}\require{cancel}
\sqrt{\dfrac{2}{5}\cdot\dfrac{5}{5}}
\\\\=
\sqrt{\dfrac{10}{(5)^2}}
\\\\=
\dfrac{\sqrt{10}}{\sqrt{(5)^2}}
\\\\=
\dfrac{\sqrt{10}}{5}
.\end{array}