ถามโค๊ดการคิดเกรดเฉลี่ยนครับ ช่วยหน่อยครับ ทำส่งอาจารย์

<?
$sbj1="Database System";
$sbj2="Network";
$sbj3="Basic Programming 2";
$sbj4="PHP";
$credit=3;
$total1=85;
$total2=70;
$total3=49;
$total4=60;

function calgrade($t){ 
if($t>=80){$grade="A"; 
}else if($t>=70){$grade="B";
}else if($t>=60){$grade="C";
}else if($t>=50){$grade="D";
}else{ $grade="F";}
return $grade;} 

?>
<table width="600" border="2" bordercolor="#FF0000">
<tr>
<td bgcolor="#99FFFF"><center><b>ลำดับที่</b></center></td> 
<td bgcolor="#FFCCFF"><center><b>ชื่อวิชา</b></center></td>
<td bgcolor="#99FFCC"><center><b>หน่วยกิต</b></center></td>
<td bgcolor="#FFCC99"><center><b>ระดับคะแนน</b></center></td>
<td bgcolor="#FFFFCC"><center><b>ระดับเกรด</b></center></td>
</tr>
<tr>
<td><center>1</td>
<td><center><?=$sbj1; ?></td>
<td><center><?=$credit; ?>
<td><center><?=$total1?></td>
<td><center><?=calgrade($total1)?></center></td>
</tr>
<tr>
<td><center>2</td>
<td><center><?=$sbj2; ?></td>
<td><center><?=$credit; ?>
<td><center><?=$total1?></td>
<td><center><?=calgrade($total2)?></center></td>
</tr>
<tr>
<td><center>3</td>
<td><center><?=$sbj3; ?></td>
<td><center><?=$credit; ?>
<td><center><?=$total1?></td>
<td><center><?=calgrade($total3) ?></center></td>
</tr>
<tr>
<td><center>4</td>
<td><center><?=$sbj4; ?></td>
<td><center><?=$credit; ?>
<td><center><?=$total1?></td>
<td><center><?=calgrade($total4)?></center></td>
</tr>
<tr>
<td colspan="4"><center><b>เกรดเฉลี่ย</b></td>
<td><center>
<?
$total = $total1 + $total2 + $total3 + $total4 / 4;
echo $total;
?>
</center></td>


</tr>
</table>

<?

$total = ($total1 + $total2 + $total3 + $total4) / 4;

echo calgrade($total);

?>

$sbj1="Database System";
$sbj2="Network";
$sbj3="Basic Programming 2";
$sbj4="PHP";
$credit=3;
$total1=85;
$total2=70;
$total3=49;
$total4=60;

function calgrade($t){ 
if($t>=80){$grade="4"; 
}else if($t>=70){$grade="3";
}else if($t>=60){$grade="2";
}else if($t>=50){$grade="1";
}else{ $grade="0";}
return $grade;} 

?>
<table width="600" border="2" bordercolor="#FF0000">
<tr>
<td bgcolor="#99FFFF"><center><b>ลำดับที่</b></center></td> 
<td bgcolor="#FFCCFF"><center><b>ชื่อวิชา</b></center></td>
<td bgcolor="#99FFCC"><center><b>หน่วยกิต</b></center></td>
<td bgcolor="#FFCC99"><center><b>ระดับคะแนน</b></center></td>
<td bgcolor="#FFFFCC"><center><b>ระดับเกรด</b></center></td>
</tr>
<tr>
<td><center>1</td>
<td><center><?=$sbj1; ?></td>
<td><center><?=$credit; ?>
<td><center>85</td>
<td><center><?=calgrade($total1)?></center></td>
</tr>
<tr>
<td><center>2</td>
<td><center><?=$sbj2; ?></td>
<td><center><?=$credit; ?>
<td><center>70</td>
<td><center><?=calgrade($total2)?></center></td>
</tr>
<tr>
<td><center>3</td>
<td><center><?=$sbj3; ?></td>
<td><center><?=$credit; ?>
<td><center>49</td>
<td><center><?=calgrade($total3) ?></center></td>
</tr>
<tr>
<td><center>4</td>
<td><center><?=$sbj4; ?></td>
<td><center><?=$credit; ?>
<td><center>60</td>
<td><center><?=calgrade($total4)?></center></td>
</tr>
<tr>
<td colspan="4"><center><b>เกรดเฉลี่ย</b></td>
<td><center>
<? 
 >>>>>>>  หมายถึงเอา              เกรด แต่ละวิชา มาบวกกันแล้วหารจำนวนวิชา   ครับ <<<<<<<<<< 
?>
</td>