ways of arranging any 4 letters chosen from the alphabet (where the order is important):We will use permutations from the previous section to see what is going on.
There are ways of arranging any 4 letters chosen from the alphabet (where the order is important):
But in this question, the order is not important. Any set of 4 letters chosen can be arranged in 4! ways.
Hence, the number of different sets of 4 letters is 
