Sum a custom field that has a shortcode in it

Without your code it’s a bit difficult, so bear with me.

Can you do this:

$x=0;

$x=do_shortcode ('[your_shortcode]');  // say its 10

$valuetoadd = 20; //for example

$exams_count_sum= $x + $valuetoadd;

echo $exams_count_sum;  //should equal 30

Or, if you are in a loop, this could be a good starting point:

$examtotal=0;   
$examtotal_sum=0;            
    while ( $the_query->have_posts() ) : $the_query->the_post();
        the_content();
        $examtotal=do_shortcode ('[your_shortcode]');
        $examtotal_sum = $examtotal_sum + $examtotal;
    endwhile;
        echo 'total is '.$examtotal_sum;  //this will be your total