MVVM update view from model

[Lightning]

At the moment I am teaching my self WPF, with the MVVM model. And I'm getting it .
But one thing I don't understand. If there is a simple view:

				<ProgressBar
				Height="18"
				Canvas.Top="89"
				Canvas.Left="14"
				Width="621" 
			Value="{Binding ProgressPercentage, Mode=TwoWay}"/>

And I have ViewModel that implements the INotifyPropertyChanged

public class ViewModelClass: INotifyPropertyChanged
{
private ModelClass mc;
public int ProgressPercentage 
{
	get { return this.mc.ProgressPercentage;}
	set 
	{
		if (this.mc.ProgressPercentage != value)
		{
			this.zah.ProgressPercentage = value;
			InvokePropertyChanged ("ProgressPercentage");
		}
	}
}
public event PropertyChangedEventHandler PropertyChanged;
private void InvokePropertyChanged(string propertyName)
 {
	 var e = new PropertyChangedEventArgs(propertyName);
	 PropertyChangedEventHandler changed = PropertyChanged;
	 if (changed != null) changed(this, e);
 }
}

And there is a simple model:

public class ModelClass 
{
public int ProgressPercentage { get;set;}
public void DoSomething()
{
    this.ProgressPercentage ++;
}
}

The model update it's %Progress, but the ViewModel and the View don't know.
If the update takes place in the ViewModel it works, because the INotifyPropertyChanged. And the Model shouldn't implement that interface (I think, because it is UI-stuff that shouldn't be in the model).
Can anyone explain how this should work?