#### Answer

$15$

#### Work Step by Step

Using $(a+b)(a-b)=a^{2}-b^{2}$, or the Distributive Property, and the properties of radicals, the given expression, $
(5+\sqrt{10})(5-\sqrt{10})
,$ simplifies to
\begin{array}{l}\require{cancel}
(5)^2-(\sqrt{10})^2
\\\\=
25-10
\\\\=
15
.\end{array}