probalo y decime si es lo que buscabas
Código:
<html>
<head><script>
function calcula(form){
a = document.getElementById("a").value;
b = document.getElementById("b").value;
c = document.getElementById("c").value;
d = document.getElementById("d").value;
e = document.getElementById("e").value;
c = parseFloat(a)* parseFloat(b)
document.getElementById("c").value = c;
e = parseFloat(c)* parseFloat(d)
document.getElementById("e").value = e;
}
</script>
</head>
<body>
<form>
<input type="text" id="a" onkeyup="calcula()">
<input type="text" id="b" onkeyup="calcula()">
<input type="text" id="c" readonly="readonly">
<input type="text" id="d" onkeyup="calcula()">
<input type="text" id="e" readonly="readonly" >
</form>
</body>
</html>