(c) The marks of the 5 student and the remainder thus they leave when divided by 3 an tabulated below.
Mark
71
76
80
82
91
Remainder
2
1
2
1
1
after the first 3 marks are entered the total has to be a multiple of 3. The remainder can only be 1, 1, 1. ∴ The 4th and 5th mark to be entered were 71 and 80.