First we are going to group the terms that contains the common factor [tex]m[/tex] in one parenthesis, and the other ones in another parenthesis:
[tex]mn-4m-5n+20[/tex]
[tex](mn-4m)-(5n+20)[/tex]
Notice that our the terms in our first parenthesis have a common factor [tex]m[/tex], and the terms in our second one have the common factor [tex]5[/tex]. Lets factor those out:
[tex]m(n-4)-5(n-4)[/tex]
Now we have a common factor [tex](n-4)[/tex] in both terms, so we can factor those out as well:
[tex](n-4)(m-5)[/tex]
We can conclude that the completely factored expression ordered alphabetically is [tex](m-5)(n-4)[/tex].
