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