diff --git a/ko-cart/index.html b/ko-cart/index.html
index d078bea..45fdc00 100644
--- a/ko-cart/index.html
+++ b/ko-cart/index.html
@@ -95,7 +95,8 @@
     <script type="text/javascript" src="js/bootstrap.js"></script>
     <script type="text/javascript" src="js/knockout-3.2.0.js"></script>
     <!-- app -->
-    <script type="text/javascript" src="js/models/product.js"></script>
+    <script type="text/javascript" src="js/models/Product.js"></script>
+    <script type="text/javascript" src="js/models/CartProduct.js"></script>
     <script type="text/javascript" src="js/viewmodel.js"></script>
   </body>
 </html>
diff --git a/ko-cart/js/models/CartProduct.js b/ko-cart/js/models/CartProduct.js
new file mode 100644
index 0000000..5a722b4
--- /dev/null
+++ b/ko-cart/js/models/CartProduct.js
@@ -0,0 +1,39 @@
+var CartProduct = function (product, units) {
+    "unit strict";
+
+    // Each cart product is composed of the product and units of the product we want to buy
+    var _product = product,
+        _units = ko.observable(units); // number of units to add or remove
+
+    var subtotal = ko.computed(function() {
+        return _product.price() * _units();
+    });
+
+    var addUnit = function () {
+        var u = _units(); // read number of units from observable
+        var _stock = product.stock();
+        if (_stock === 0 ) {
+            return;
+        }
+        _units(u + 1); // assign observable with new unit
+        _product.stock(--_stock); // reduce inventory of stock by decrease unit from product
+    };
+
+    var removeUnit = function () {
+        var u = _units();
+        var _stock = _product.stock();
+        if (u === 0) {
+            return;
+        }
+        _units(u - 1);
+        _product.stock(++_stock);
+    };
+
+    return {
+        product: _product,
+        units: _units,
+        subtotal: subtotal,
+        addUnit: addUnit,
+        removeUnit: removeUnit
+    }
+}
\ No newline at end of file
diff --git a/ko-cart/js/models/product.js b/ko-cart/js/models/Product.js
similarity index 100%
rename from ko-cart/js/models/product.js
rename to ko-cart/js/models/Product.js
diff --git a/ko-cart/js/viewmodel.js b/ko-cart/js/viewmodel.js
index 889e3b3..8e88005 100644
--- a/ko-cart/js/viewmodel.js
+++ b/ko-cart/js/viewmodel.js
@@ -47,6 +47,24 @@ var vm = (function () {
       $("#cartContainer").removeClass("hidden");
     }
   };
+  var addToCart = function(data) {
+    var item = null;
+    var tmpCart = cart();
+    var n = tmpCart.length;
+    while(n--) {
+      if (tmpCart[n].product.id() === data.id()) {
+        item = tmpCart[n];
+      }
+    }
+    if (item) {
+      item.addUnit();
+    } else {
+      item = new CartProduct(data, 0);
+      item.addUnit();
+      tmpCart.push(item);
+    }
+    cart(tmpCart);
+  };
 
   return {
     // first chapter