รบกวนสอบถามการหาค่าเฉลี่ย ของเกรดครับ พอแสดงค่าแล้วนำมารวมยังไงคับ

 <? 
					
					$sqlYea="SELECT Yeae_ID FROM yearedu WHERE Yeae_End >= '".date("Y-m-d")."' ";
					$resultYea = $mysqli->query($sqlYea);
    				$resultYea && $resultYea->num_rows>0;
					$rowYea = $resultYea->fetch_assoc();
					$rowYea["Yeae_ID"];
					$sql="SELECT * FROM schedule WHERE Cla_ID =$_REQUEST[Cla_ID] AND Roo_ID =$_REQUEST[Roo_ID]  and Yeae_ID='".$rowYea["Yeae_ID"]."' group by SubJ_Code ORDER by SubJ_Code ASC";
					$result = $mysqli->query($sql);
					if($result && $result->num_rows>0){ 
						while($row = $result->fetch_assoc()){ 
								$sqlSu="SELECT SubJ_Name FROM subject WHERE SubJ_Code = '$row[SubJ_Code]' ";
									$resultSu = $mysqli->query($sqlSu);
									$resultSu && $resultSu->num_rows>0;
									$rowSu = $resultSu->fetch_assoc();
									$sqlSu1="SELECT Ssc_Sco1,Ssc_Sco2 FROM stuscores WHERE Stu_Code = $_REQUEST[Stu_Code] and SchD_ID=$row[SchD_ID]  and Tem_ID='1' ";
									$resultSu1 = $mysqli->query($sqlSu1);
									$resultSu1 && $resultSu1->num_rows>0;
									$rowSu1 = $resultSu1->fetch_assoc();
									$sqlSu2="SELECT Ssc_Sco1,Ssc_Sco2 FROM stuscores WHERE Stu_Code = $_REQUEST[Stu_Code] and SchD_ID=$row[SchD_ID]  and Tem_ID='2' ";
									$resultSu2 = $mysqli->query($sqlSu2);
									$resultSu2 && $resultSu2->num_rows>0;
									$rowSu2 = $resultSu2->fetch_assoc();
							 	 $score=$rowSu1[Ssc_Sco1]+$rowSu1[Ssc_Sco2]+$scoress=$rowSu2[Ssc_Sco1]+$rowSu2[Ssc_Sco2];
								
										if( $score >= 80 ){
											 $total =4;
										}else if( $score >= 75 && $score <= 79 ){
											$total =3.5;
										}else if( $score >= 70 && $score <= 74 ){
											 $total =3;
										}else if( $score >= 65 && $score <= 69 ){
											 $total =2.5;
										}else if( $score >= 60 && $score <= 64 ){
											 $total =2;
										}else if( $score >= 55 && $score <= 59 ){
											 $total =1.5;
										}else if( $score >= 50 && $score <= 54 ){
											 $total =1;
										}else{
											 $total =0;
										} 
							$total;
							$SumTotal = $SumTotal + $total.'/n';
						}
						 $a=array($SumTotal);
			$b=  array_sum($a);
                        echo $b/10;
						}  
						?>

echo $b/10;//ทำไมตรงเลข 10 ไม่ namrow จำนวนที่นำมาคำนวนด้วยเลยครับ เพื่อมีการเพิ่มลดวิชา คิดเห็นเฉยๆ นะ
// $b/$a; อะไรประมาณนั้น