Who said that "nofollow" links are not help in SEO ranking or it was official announcement from Google?
Here is your answer:
[video=youtube;g37bwBlifnk]https://www.youtube.com/watch?v=g37bwBlifnk[/video]
Pay close attention to what Matt Cutts says at the beginning of that video. He says
"Nofollow links are dropped out of Google's link graph. So, they are irrelevant from a search engine's point of view."
He is telling you right then and there (straight from the mouth of the guy who heads the webspam team at Google) that nofollow links are irrelevant.
They don't count. Period.
Do you think that just the organic natural link and dofollow links are considered as helping in SEO?
The only links that are going to help you rank better in Google are links that Google trusts and counts. If there is a good chance that Google won't trust a link then there is a good chance that Google won't count the link. (Maybe it will and maybe it won't.)
How do you know if Google will trust the link for sure or not?
You ultimately don't know until after you get the link. Even after getting a link sometimes you can't tell if that link is helping or not. When you are deciding about whether to try to get a certain link, you just make your best judgement call based on common sense, logical reasoning and intuition. If its going to be a dofollow link that looks legit (looks like it was created by the webmaster/author because the link is useful) and the link comes from a clean, useful website (useful website in good neighborhood that isn't spammy and doesn't post link spam) then it is probably going to count. The amount of rank boosting power it has is determined by the amount of Google PageRank flowing through the link that points to your page. It is also determined by the anchor text of the hyperlink pointing to your page.
How many percents of "nofollow" and "dofollow" of a site is good for SEO?
Nofollow links are irrelevant as Matt Cutts already said. That means, it doesn't matter one bit how many nofollow links your site/page has pointing to it. In other words, you can have zero inbound nofollow links and it just won't matter. They aren't affecting your rankings anyway. You can have 100% nofollow links and it won't matter. They aren't affecting your rankings anyway. The only links that affect your ranking are links that are counted by Google. Nofollow links are not counted. Therefore they are completely and totally irrelevant. Ignore them. Don't bother building them at all if you are building them for SEO purposes. It is a giant waste of your time.
I hope that was clear enough?