Airports are identified with 3-letter codes. For example, the richmond, virginia airport has the code ric, and portland, oregon has pdx. How many different 3-letter codes are possible?

There are, you see, 3 x 2 x 1 = 6 possible ways of arranging the three digits. Therefore in that set of 720 possibilities, each unique combination of three digits is represented 6 times. So we just divide by 6. These are what are mathematically called "combinations".