The number of trees that Amiyah cut is 20/100*400 = <<20/100*400=80>>80
The number of trees remaining on the farm after she cut 80 trees is 400-80 = <<400-80=320>>320
For every tree that Amiyah cut, she plants 5 new trees, and since she cut 80 trees, she planted 80*5 = <<80*5=400>>400 new trees.
The total number of trees on the farm is now 320+400 = <<320+400=720>>720 trees.
#### 720