This code works, but not like I want it to work. Now it is taking the value to open, I want it to use the id to open. I can't use "Ja" constantly as value because i have multiple value's "Ja"(not shown in the code below).
    $( document ).ready( function () {
      $( "input[name=group8" ).change( function() {
        var test8 = $( this ).val();
        $( ".desc" ).hide();
        $( "#" + test8 ).show();
      } );
    } );.desc {
      display: none;
    }<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<form name="form10" id="my_form10" method="post" action="" >
      <h5>Wil je keukenplinten laten inkorten na het leggen van de vloer?</h5>
      <div class="radio8">
        <div class="janee">
          <input id="JA7" type="radio" name="group8" value="Ja">
          <label for="JA7" class="form-field__radio__label">Ja, meerprijs € 20.00 per plint</label><br>
          <input id="NEE7" type="radio" name="group8" value="Nee">
          <label for="NEE7">Nee</label>
        </div>
      </div>
    </form>
    
    <form id="keukenplinten">
      <div id="Ja" class="desc" style="float: inherit;">
        <h5>Hoeveel keukenplinten wil je laten inkorten?</h5>
        <input type="number" id="keukenplintenInkorten" name="keukenplinten">
      </div>
    </form> 
     
     
     
     
     
     
    