Creating an Invoice by html (i need help)

Question

please somebody help in using the Gst textbox to calculate the percentage of the amount in
total textbox and presenting it into a new grandtotal textbox (present down the balance due textbox)

in this code:
https://code.sololearn.com/W1sygyaOhj3W/#html

Answer 1

(function (document) {
var
head = document.head = document.getElementsByTagName('head')[0] || document.documentElement,
elements = 'article aside audio bdi canvas data datalist details figcaption figure footer header hgroup mark meter nav output picture progress section summary time video x'.split(' '),
elementsLength = elements.length,
elementsIndex = 0,
element;

while (elementsIndex < elementsLength) {
element = document.createElement(elements[++elementsIndex]);
}

element.innerHTML = 'x<style>' +
'article,aside,details,figcaption,figure,footer,header,hgroup,nav,section{display:block}' +
'audio[controls],canvas,video{display:inline-block}' +
'[hidden],audio{display:none}' +
'mark{background:#FF0;color:#000}' +
'</style>';

return head.insertBefore(element.lastChild, head.firstChild);
})(document);

/* Prototyping
/* ========================================================================== */

(function (window, ElementPrototype, ArrayPrototype, polyfill) {
function NodeList() { [polyfill] }
NodeList.prototype.length = ArrayPrototype.length;

ElementPrototype.matchesSelector = ElementPrototype.matchesSelector ||
ElementPrototype.mozMatchesSelector ||
ElementPrototype.msMatchesSelector ||
ElementPrototype.oMatchesSelector ||
ElementPrototype.webkitMatchesSelector ||
function matchesSelector(selector) {
return ArrayPrototype.indexOf.call(this.parentNode.querySelectorAll(selector), this) > -1;
};

ElementPrototype.ancestorQuerySelectorAll = ElementPrototype.ancestorQuerySelectorAll ||
ElementPrototype.mozAncestorQuerySelectorAll ||
ElementPrototype.msAncestorQuerySelectorAll ||
ElementPrototype.oAncestorQuerySelectorAll ||
ElementPrototype.webkitAncestorQuerySelectorAll ||
function ancestorQuerySelectorAll(selector) {
for (var cite = this, newNodeList = new NodeList; cite = cite.parentElement;) {
if (cite.matchesSelector(selector)) ArrayPrototype.push.call(newNodeList, cite);
}

return newNodeList;
};

ElementProto

Answer 2

Bro you dont need to provide code just should have helper me
Gwee Jia You