Rob hunts 1/2 * 6 animals/day = <<1/2*6=3>>3 animals/day.
Rob and Sam hunt 3 animals/day + 6 animals/day = <<3+6=9>>9 animals/day.
Mark hunts 1/3 * 9 animals/day = <<1/3*9=3>>3 animals in a day.
Peter hunts 3 * 3 animals/day = <<3*3=9>>9 animals in a day.
The total they all hunt in a day is 6 animals/day + 3 animals/day + 3 animals/day + 9 animals/day = <<6+3+3+9=21>>21 animals.
#### 21