If Johnny had half as much practice as he currently has as of 20 days ago, this means he has 20*2=<<20*2=40>>40 days worth of practice currently.
Therefore, to have three times as much practice as he currently has, he'd need to practice for 40*3=<<40*3=120>>120 days.
Since he's already practiced for 40 days, this means he needs to practice or 120-40= <<120-40=80>>80 more days
#### 80