<form enctype="multipart/form-data" method="post" id="form">
<textarea placeholder="Введите число" name="num" rows="1" cols="45" required></textarea></br>
<p><input value="Отправить" type="submit"></p></form>
<div id="content"></div>

<script>
$(function(){
  'use strict';
$('#form').on('submit', function(e){
    e.preventDefault();
    var fd = new FormData( this );
    $.ajax({
      url: 'sen.php',
      type: 'POST',
      contentType: false,
      processData: false,
      data: fd,
      success:function(html){
$("#content").html(html);
 }
    });
  });
});
</script>

<?php
$num = $_POST['num'];
for ($i = 1; $i < 4; $i++) {
    echo $i*$num."<br/>";
    sleep(1);
  }
?>