I finished every level in about 5 seconds (except for 31 -for some reason I just couldn't get it-) until about level 35. Then you really have to think as there are multiple branches.
The only problem I found was the secret level wouldn't accept my correct answer. I finished it 3 times and nothing. Finally, I watched the video and it SEEMS you must places the tiles in a certain 'sequence'... 1,2,3,4,5,6,11,9,12,8,7,10... Very odd, but finally the game registered the answer. Other than that weird glitch the game was great.