"That which you persist in doing becomes easy to do - not that the nature of the thing has changed, but your power and ability to do has increased." -- H.J. Grant
In the menu choice just above, I have created each answer to the example problems as a GIF. If you feel you're getting the idea by the time you get to mass-mass, turn off image loading, download and try several problems and then take a look at the answers one-by-one.