function comp(){
var num = document.getElementById("num").value;
var discount = document.getElementById("discount").value;
var price = document.getElementById("price").value;
var recAmount = num * price * discount;
document.getElementById("recAmount").value=recAmount;
}
<html>
<head></head>
<body>
number: <input type="text" id="num" onkeyup="comp();" /><br/>
discount:<input type="text" id="discount" onfocus="this.blur();" value="0.6" /><br/>
price: <input type="text" id="price" onfocus="this.blur();" value="100" /><br/>
recAmount: <input type="text" id="recAmount" onfocus="this.blur();" /><br/>
<script type="text/javascript">
function comp(){
var num = document.getElementById("num").value;
var discount = document.getElementById("discount").value;
var price = document.getElementById("price").value;
var recAmount = num * price * discount;
document.getElementById("recAmount").value=recAmount;
}
</script>
</body>
</html>
