Multiplication is not working in methods

i am having issues with my vuejs code, i am new to the vuejs i am trying to multiply dynamically. in qntrate = total
as 4
5=20 but doesn’t work as the expected rather gives 0(45=0) but works when 45a=20 where a is the numeric integer

here is the codepane link:-https://codepen.io/razev23/pen/zQVjRj?editors=1010