How do I calculate the selected value for "N/a" in rating, which means this rows is not count?

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I have create code to calculate the average of hod score. How do I get the average of score if the user select "N/A", which means the whole rows did not count.

I create a simple formula:-
If user choose rate 5 for 4 question, the final average score will be 5 (5+5+5+5/4).
If user choose rate 5 for 4 question, but 1 question they choose "N/A", means the final score will be 5 too (5+5+5/3), so the other 1 question did not included.

What I have tried:

PHP Code

$ListRate = array(0 => "Please Select", 5 => "5",  4 => "4", 3 => "3", 2 => "2", 1 => "1", 0 => "N/A");

$mySQL2 = 'SELECT A.final_average_score, AVG(B.hod_score) as final_average_score, A.total_score FROM assessment_technical A INNER JOIN assessment_technical_sectiona B WHERE B.assessment_technical_id = '.$data['id'];
$result2 = mysql_query($mySQL2);
	if ($result2){
		if (mysql_num_rows($result2) > 0){
			$row2 = mysql_fetch_assoc($result2);
			$data['final_average_score'] = $row2['final_average_score'];
			$data['total_score'] = $row2['final_average_score']/5*100*0.3;
		}
	}

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