web-master,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
</style>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
<script>
$(function() {
$("input[name='percent']").change(function() {
var name = $.trim(this.value);
$("#period").prop("required", !!name)
})
});
</script>
</head>
<body>
<form action="http://" >
<input name="percent" type="tel" style="text-align:center !important;" value="" title="Вводите только цифры" pattern="^[0-9\,\.]+$" placeholder="Проценты" data-wrapper-class="controlgroup-textinput ui-btn">
<select name="period" id="period">
<option value=""></option>
<option value="1">Сутки</option>
<option value="2">Неделя</option>
<option value="3">Месяц</option>
<option value="4">Год</option>
</select>
<input name="" type="submit" value="ok"></form>
</body>
</html>