вот это ты наворочил
<html>
<head>
</head>
<body>
<input type="number" min="0" id="one">
<input type="number" min="0" id="who">
<select id="three">
<option value="2190.94">USD 2190.94</option>
<option value="121712.36">RUB 121712.36</option>
<option value="1828.38951">EUR 1828.38951</option>
</select>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<script>
var one = $('#one');
var who = $('#who');
var three = $('#three');
one.on('input', function() {
who.val(one.val()*three.val());
});
who.on('input', function() {
one.val(who.val()/three.val());
});
three.on('change', function() {
who.val(one.val()*three.val());
});
</script>
</body>
</html>