JSFiddle - React, Tailwind, and code Playground

HTML

<label>Qty:</label>
<input name="qty" id="qty" type="text" />
<input name="price" id="price" type="hidden" value="30" /><br />

<label>Total:</label>
<input name="total" id="total" type="text" />

JavaScript

$(document).ready(function(){
    var qty=$("#qty");
    qty.keyup(function(){
        var total=isNaN(parseInt(qty.val()* $("#price").val())) ? 0 :(qty.val()* $("#price").val())
        $("#total").val(total);
    });
});