JSFiddle - React, Tailwind, and code Playground

HTML

<input id="quantity" size="30" type="number" value="1"/>
 <p id="total" value="29.99">29.99</p>

JavaScript

$(document).ready(function() {
    function updatePrice() {
        var num = parseInt($("#quantity").val());
        var price = (num) * parseFloat($("#total").text());
        var total = price.toFixed(2);
        $("#total").replaceWith(total);
        $('body').append(price); 
    }
    $(document).on("change, mouseup, keyup", "#quantity", updatePrice);
});