JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://code.jquery.com/mobile/1.0b2/jquery.mobile-1.0b2.min.css">
<div data-role="page" id="home"> 
    <div data-role="content">
 
        <label for "rabat">Rabat:</label><input id="rabat" type="number" value="5"/><br>
        <label for "cenastart">Cena wyjsciowa:</label><input id="cenastart" type="number" value="120.99"/><br> 
        <label for "cena">Cena po rabacie:</label><input id="cena" type="number" />  
          
        <p><button id="btn1">Przelicz rabat</button></p> 
    </div>
</div>

JavaScript

$("#btn1").click(function() {
     rabat = parseInt($('#rabat').val());
     
     cenastart = parseFloat($('#cenastart').val());
     
     cena = cenastart + (rabat/100) * cenastart;
     
     cena = Math.round(cena *100) / 100;
     
     $('#cena').val(cena);
 });