ng-change

HTML

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<body ng-app="SimpleInterestApp" ng-controller="simpleCtrl">
  <div class="container">
    <div class="row">
      <input type="text" ng-change="onChange()" ng-model="P" class="col-xs-4 col-sm-4 col-md-4 col-lg-4" placeholder="Principle" />
      <input type="text"  ng-model="R" class="col-xs-4 col-sm-4 col-md-4 col-lg-4" placeholder="Rate" />
      <input type="text" ng-change="onChange(this.T,'T')" ng-model="T" class="col-xs-4 col-sm-4 col-md-4 col-lg-4" placeholder="Time" />
    </div>
  </div>
</body>

JavaScript

angular.module('SimpleInterestApp', [])
  .controller('simpleCtrl', function($scope) {
    $scope.onChange = function() {
      $scope.R="";
    };

    $scope.isNumeric = function(n) {
      return !isNaN(parseFloat(n)) && isFinite(n);
    };


  });