I don't understand...
There are 10 zoots that can be achieved in 3 difficulties, that means 30 zoots in all. 15 zoots should be the 50%.
I got 6 normal zoots and 2 easy zoots
6 * 2 + 2 * 1 = 14
but my progress rate is 36%... mh...
Let me explain
There are really only 10 zoots. They can be completed on one of 3 difficulties but only the hardest difficulty you have achieved counts.
The amount the Zoot is worth depends on what difficulty you have completed it on (Easy=3%, Normal=5%, Hard=10%). You can see from that, completing all Zoots on Hard will give you 100% even if you've never done a Zoot on Easy or Normal.
So in your case:
6 x Normal = 6 x 5% = 30%
2 x Easy = 2 x 3% = 6%
Total Progress = 30% + 6% = 36%
In reality it's a lot simpler than it might sound.