Math
posted by BWB .
For the threeletter set {R, S, T}, find the twoletter combinations.
I realize that this deals with combinations and not permutations. I just do not understand how they want this done.

you want 2 letters at a time, order not important
so
RS
RT
ST
there are only 3 cases
in combination notation
C(3,2) = 3!/(2!1!) = 6/2 = 3