There were 80/2 = <<80/2=40>>40 wireless mice sold.
There were 80/4 = <<80/4=20>>20 optical mice sold.
So, there were a total of 40 + 20 = <<40+20=60>>60 wireless and optical mice sold.
Therefore, 80 - 60 = <<80-60=20>>20 trackball mice were sold this month.
#### 20