Reynaldo, I’m working up the answers and justifications for those problems you mentioned above. It looks like you missed one question in each of the DMAIC sections. 21/25 is pretty good – 84%!
However, with just 25 questions it’s difficult to call out one section as needing more improvement than another. Try taking the next free online quiz and let me know how you scored. The link you should use is here: http://asq.org/six-sigma-training/greenbelt/ready.html