Let's calculate the total marks obtained by the six students.
The first two students obtained a mark of 73%, which gives us a total of 73 + 73 = 146 marks.
The next three students obtained a mark of 68% each, which gives us a total of 68 + 68 + 68 = 204 marks.
The average score obtained by the group is 71%. Since there are six students, the total marks obtained by all six students would be 6 * 71 = 426 marks.
To find the mark obtained by the sixth student, we can subtract the total marks obtained by the first five students from the total marks obtained by all six students: 426 - (146 + 204) = 76 marks.
Therefore, the mark obtained by the sixth student is 76%.