How to add a percentage to the total?

Question

opencart

here in the photo you can see he adds 10 rubles to the total amount, and I need to make it not 10 rubles, but 10%

Answer 1

$prc=$sum*0.1;
$total=$sum*1.1; OR $total=$sum+$prc;

Answer 2

Correct the calculation in the module. Can help