>>>Bruce, I didn't use combinatorics to find solution. There is another purely algebra method to do this.
>>
>>It's probably easier, too. My training has me using combinations/factorials as a medium for deriving solutions for this type of problem, but it's certainly not always the best/easiest way. Though the fastest way may be to guess the solution and use induction to show it works :~)
>
>In math analysis, this problem is actually finding a line crossing 4 points.

Ah, like best-fitting a polynomial?