If they ate 2/3rds, that leaves 1/3. So 1/3rd of 16 is 16/3 slices. 16/3 slices divided between 3 people is 16/9rds slices apiece
If the slices is not 16, it will make sense. This is 12 slices
If they ate 2/3rds, that leaves 1/3. So 1/3rd of 12 is 4 slices. 4 slices divided between 3 people is 4/3rds slices apiece