JSFiddle - React, Tailwind, and code Playground

HTML

<input type="checkbox"  value="4" id="in"/>
<select id="inp" >
<option value="0">0</option>
<option value="10">10</option>
<option value="11">11</option>
<option value="12">12</option>
</select>
<div id="vivod"></div>

JavaScript

$(document).ready(function() {
var chislo = 2;
    
    function result(){
       var check=$('#in'),
           sel_value=$('#inp');
       $(" #vivod ").text(
           (check.prop('checked')?check.prop('value')*1:0)+
            sel_value.val()*chislo
        );
    }
    
   $(" #inp ").change(result);
   $('#in').change(result).trigger('change'); 
});