If David has 15 shells, then Mia has 4 *15 = <<15*4=60>>60 shells.
Ava has 20 more shells than Mia, so that is 60+20 = 80 shells.
Alice has 1/2 the number of shells Ava has, which is 1/2*80 = <<1/2*80=40>>40 shells.
The total number of shells all of them have is 15+60+80+40 = <<15+60+80+40=195>>195 shells.
#### 195