all, (lesson time :-P)
nCr 同或然率差唔多, C=combination, n = total number of parameter, r=selected parameter. Eg. There is five balls numbered A, B, C, D, E. If I select 3 balls each time, there is total 10 combination law (1. A,B,C; 2.A,B,D; 3.A,B,E; 4. B,C,D; 5. B,C,E; 6. C,D,E; …….). The formula to calculate the total combination is nCr [ie n!/r!(n-r)!]