JSFiddle - React, Tailwind, and code Playground

HTML

<form name="form1">
 <table width="200" border="1" cellpadding="3" cellspacing="0">
  <tr>
   <td align="center" style="height:50px;">
    <select id="ders">
     <option value="12,5">Zumba</option>
     <option value="10">Pilates</option>

    </select>
   </td>
  </tr>
  <tr>
   <td align="center" height="35" valign="bottom">JQuery İle</td>
  </tr>
  <tr>
   <td align="center"><input type="text" value="12,5" name="textfield4" id="textfield4"></td>
  </tr>
     
 </table>
</form>

JavaScript

// JQuery İle
$(function() {
    $('ders').on('change',function() {

        $('#textfield4').val($(this).val());
    });
});