javascript is a interpreter language ,so you need to use different names for element.
txtFirstNumber..txtFirstNumber1,txtSecondNumber1...
<html>
<head>
<title>
Setting up variables in JavaScript
</title>
<SCRIPT type = "text/JavaScript"> //language="javascript" it is outdated //and should be considered as poor coding practice.
A = 500;
B = 1500;
C = B+A;
alert(C);
function calculate() {
A = parseInt(document.frmOne.txtFirstNumber.value);
B = parseInt(document.frmOne.txtSecondNumber.value);
C = A+ B
document.frmOne.txtThirdNumber.value=C;
}
//parseInt(document.getElementById("value1").value);
function calculate1() {
A = parseInt(document.frmOne.txtFirstNumber1.value);
B = parseInt(document.frmOne.txtSecondNumber1.value);
C = parseInt(document.frmOne.txtThirdNumber1.value);
D = ((A * B)* C )/ 100
document.frmOne.txtFourthNumber.value = D;
}
</script>
<body>
<FORM NAME = frmOne>
Number One: <INPUT TYPE = Text NAME = txtFirstNumber SIZE = 5 value =""> <br /><br />
Number Two: <INPUT TYPE = Text NAME = txtSecondNumber SIZE = 5 value ="">
<P>
Total: <INPUT TYPE = Text NAME = txtThirdNumber SIZE = 5 value = What?>
<p>
<Input Type = Button NAME = b1 VALUE = "Add Numbers" onClick = "calculate();">
<br /><br />
Number One: <INPUT TYPE = Text NAME = txtFirstNumber1 SIZE = 5 value ="">
<br /><br />
Number Two: <INPUT TYPE = Text NAME = txtSecondNumber1 SIZE = 5 value ="">
<br /><br />
Number Three: <INPUT TYPE = Text NAME = txtThirdNumber1 SIZE = 5 value ="">
<P>
Total: <INPUT TYPE = Text NAME = txtFourthNumber SIZE = 5 value = "aha">
<P>
<Input Type = Button NAME = b1 VALUE = "Add Numbers" onClick = "calculate1();">
</FORM>
</body>
</head>
</html>
