Paint the windows one at a time. There are 4 colours you can paint the first window, and 4 you can paint the second, and the third etc. so the number of combinations is by the multiplication principle.
There are 6 windows and you want to paint them. There are 4 colors to choose from, but you cannot mix the colors the windows. (each window must have a unique color)
How many different combinations can you paint the windows?
I think the answer is 3 x 4! or 6 x 4! but something doesn't feel right