```Jscript
function addText(txt)
{
document.getElementById("input_text").value+=txt;
}
function clearText(txt)
{
document.getElementById("input_text").value="";
}
function removelastText()
{
var strng=document.getElementById("input_text").value;
document.getElementById("input_text").value=strng.substring(0,strng.length-1);
}
function answerText()
{
//easiest code "eval()"
try {
document.calc.input_text.value = eval(document.calc.input_text.value);;
} catch (e) {
if (e instanceof SyntaxError) {
alert(e.message);
}
}
function allowedChar(elem) {
var validChars = /[0-9(). ]/;
var strIn = elem.value;
var strOut = '';
for(var i=0; i < strIn.length; i++) {
strOut += (validChars.test(strIn.charAt(i)))? strIn.charAt(i) : '';
}
elem.value = strOut;
}
```