There are 9 different colors of paint to choose from. Out of the 9 colors, how many ways can 4 different colors be chosen?
Think about choosing 4 colours, one at a time.
There are 9 choices for the first colour. The colours have to be different, so that one cannot be chosen again, 8 colours are left.
There are 8 choices for the second colour, and in the smae way, 7 choices for the third colour and 6 choices for the fourth colour.
The total number of possibilities is:
This can also be written as
Note why this works: