If I am understanding you question right, it should be pretty simple.... try something like this:
for ($i = 0; $i < $recordCount; $i++) {
print("<tr>\n\t<td>".$studentData[$i][0]."</td>\n");
print("\t<td>".$studentData[$i][1]."</td>\n");
print("\t<td><center>".$studentData[$i][2]."</center></td>\n");
print("\t<td><center>".$studentData[$i][3]."</center></td>\n");
print("\t<td><center>".$studentData[$i][4]."</center></td>\n");
$studentData[$i][5] = (round((($studentData[$i][2] * 30) / 30) + (($studentData[$i][3] * 40) / 45) + (($studentData[$i][4] * 30) / 65)));
print("\t<td><center>".$studentData[$i][5]."</center></td>\n");
if ($studentData[$i][5] <= 49) {
print("\t<td><center>N</center></td>\n");
} elseif (($studentData[$i][5] >= 50) AND ($studentData[$i][5] <= 59)) {
print("\t<td><center>C</center></td>\n");
} elseif (($studentData[$i][5] >= 60) AND ($studentData[$i][5] <= 69)) {
print("\t<td><center>CR</center></td>\n");
} elseif (($studentData[$i][5] >= 70) AND ($studentData[$i][5] <= 79)) {
print("\t<td><center>D</center></td>\n");
} elseif (($studentData[$i][5] >= 80) AND ($studentData[$i][5] <= 100)) {
print("\t<td><center>HD</center></td>\n");
}
}