[BruceZ]

It is just (13,2) = 13*12/2 = 78. That is, there are 13 ways to pick the first rank, and 12 ways to pick the second rank, and then you divide by 2 because otherwise you would count every hand twice since you can get the cards in either order (like J,T or T,J are the same hand). You ignore suits entirely and just deal with 13 numbers. This is the same as 13*(4,2) = 13*4*3/2 = 13*12/2 = 78. I can't think of why anyone would come up with that right off.