<!DOCTYPE html>
<html ng-app = "myApp">

  <head>
    <script data-require="angular.js@1.3.15" data-semver="1.3.15" src="https://code.angularjs.org/1.3.15/angular.js"></script>
    <link rel="stylesheet" href="style.css" />
    <script src="script.js"></script>
  </head>

  <body>
    <div class="keypadcontainer" ng-controller="anodekeypadcntrl">
  ------
  <input type="button" value="24" id="twentyFour" ng-class =" active"
	ng-click="clickTwentyFour($event)" />
  -------------
</div>
  </body>

</html>
// Code goes here
var app = angular.module('myApp', []);
app.controller('anodekeypadcntrl',function($scope){
	
	
	$scope.clickTwentyFour = function(event){
	  //In this function I need to highlight the button (adding the .active class to it)	
	  $scope.active = "active"
	};
	                            
});
/* Styles go here */

.active {
  background: red !important;
}