*** Combinations of 4 items in a group of 10 There are 210 (red tile position 10 - 4) different combinations (when the order does not matter) of 4 items in a group of 10. There are 715 (green tile) different combinations with repetitions of 4 items in group of 10. #### This is called combination (or k-combination) in mathematic, id est no matter of the order of the elements and no repetition of elements. The formula is the binomial coefiicent one. n! C(n,k) = ---------- k!(n-k)! #### perl -E "say @$_ for grep{$$_[0]!=$$_[1] and $$_[0]!=$$_[2] and $$_[0]!=$$_[3] and $$_[1]!=$$_[2] and $$_[1]!=$$_[3] and $$_[2]!=$$_[3]} map { [split '',sprintf '%04s',$_]} 0..9999;"