#### Answer

$(4+m)(5+3n)$

#### Work Step by Step

$\bf{\text{Solution Outline:}}$
Factor by grouping the first $2$ and the last $2$ terms of the given expression, $
20+5m+12n+3mn
.$ Then, factor the $GCF$ in each group. Finally, factor the $GCF$ of the entire expression.
$\bf{\text{Solution Details:}}$
Grouping the first $2$ and the last $2$ terms, the given expression is equivalent to
\begin{array}{l}\require{cancel}
(20+5m)+(12n+3mn)
.\end{array}
Factoring the $GCF$ in each group, results to
\begin{array}{l}\require{cancel}
5(4+m)+3n(4+m)
.\end{array}
Factoring the $GCF=
(4+m)
$ of the entire expression above results to
\begin{array}{l}\require{cancel}
(4+m)(5+3n)
.\end{array}