JSFiddle - React, Tailwind, and code Playground

HTML

<form name="frm">


        <label>cost: <input type="hidden" name="200" size="5" class="summable" value="200"/></label> 
        qty:     <select name="qty">
    <option value="0">0</option>
    <option value="1">1</option>
    <option value="2">2</option>
    <option value="3">3</option>
    <option value="4">4</option>
    <option value="5">5</option>
    </select><br/>

     

        <strong id="summation"></strong><br/>
    </form>

JavaScript

function doTotal() {
    var total = 0.0;

    $(".summable").each(function () {
    $this = $(this);

        var cost = parseFloat($this.val());
        var qty_selector = '[name=' + 'qty' + ']'; // eg: [name=cost_3_qty]
        var qty = parseFloat($(qty_selector).val());
        if (cost && qty)
            total = cost * qty ;});

    $("#summation").html(total.toFixed(4));
}

$(document).ready(function() {
   
        
    $('select').change(function() {
        doTotal();        
    });
});