Factor mn - 4m - 5n + 20

Mn - 4m - 5n + 20

In this, mn and - 4m have a common factor; m. So we can take that out to make it:

m (n-4) - 5n + 20

Now, - 5n + 20 also has a common factor which is 5, because 20/5 gives you 4, and 5/5 equals 1. So we can talk out that - 5 (it doesn't need to be negative, but negatives on variable, no gusta). So you'd get

m (n-4) - 5 (n-4) OR m (n-4) + 5 (-n+4)