How many subsets of three letters each does the set a b c d e f have?

The set {a, b, c, d, e, f} contains 6 letters. To find the number of subsets of three letters, we can use the combination formula ( \binom{n}{r} ), where ( n ) is the total number of items, and ( r ) is the number of items to choose. Here, ( n = 6 ) and ( r = 3 ), so the calculation is ( \binom{6}{3} = \frac{6!}{3!(6-3)!} = 20 ). Therefore, there are 20 subsets of three letters each.