<!doctype html>
<html>
<head>
<meta charset="utf-8" />
<title></title>
<style>
p { margin: 0; }
</style>
</head>
<body>
<label for="sum">Введите сумму: </label>
<input type="text" placeholder="0" id="sum" /><br />
<p>3% <span id="sum1">0</span></p>
<p>15% <span id="sum2">0</span></p>
<script>
var sum = document.getElementById('sum'),
sum1 = document.getElementById('sum1'),
sum2 = document.getElementById('sum2');
sum.addEventListener('input', function() {
this.value = this.value.replace(/\D/g, '');
sum1.innerHTML = Math.round( this.value * 1.03 );
sum2.innerHTML = Math.round( this.value * 1.15 );
});
</script>
</body>
</html>