Let's examine a random arrangement of 6 "t"s and 4 "-"s: Suppose we look at this arbitrary arrangement: - - t t - t t t - t Let's number the positions: 1 2 3 4 5 6 7 8 9 10 - - t t - t t t - t Notice that there are 10 positions. This particular one chooses positions 1,2,5, and 9 for the "-"s and the rest has "t"'s So there are 10 positions, and we choose 4 of them to put the -'s. That's "10 choose 4" or 10C4 or C(10,4) or= = 210 ways Edwin