Answer
$15$
Work Step by Step
$\bf{\text{Solution Outline:}}$
To simplify the given radical expression, $
(\sqrt{20}-\sqrt{5})(\sqrt{20}+\sqrt{5})
,$ use the special product on multiplying the sum and difference of like terms.
$\bf{\text{Solution Details:}}$
Using the product of the sum and difference of like terms which is given by $(a+b)(a-b)=a^2-b^2,$ the expression above is equivalent
\begin{array}{l}\require{cancel}
(\sqrt{20})^2-(\sqrt{5})^2
\\\\=
20-5
\\\\=
15
.\end{array}