mn - 4m - 5n + 20
Group the terms as follows:
(mn - 4m) + (-5n + 20)
Take m as a common factor in the first parentheses and -5 in the second one.
m*(n - 4) - 5*(n - 4)
Take (n - 4) as a common factor.
(n - 4)*(m - 5)
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