Use JavaScript.
Eg.
<form name="example">
<input type="text" size="20" name="calculator">
<input type="button" name="B1" value="Calculate" onclick="cal()">
<input type="reset" name="B2" value="Reset">
Answer:<input type="text" size="20" name="answer">
</form>
<script>
function cal()
{
document.example.answer.value=
eval(document.example.calculator.value)
}
</script>
Now see the Form name is "example"
So u can access the textbox value as
"document.example.answer.value"
Gud Luck !