The solution is always easy, you just have to find it.
Cast Donuts has beautifully shaped rings that look like 2 donuts intertwined. Try solving the puzzle by imagining the inner structure and using each of the rings’ division lines as a hint. You will be pleasantly surprised by the unique movements needed to separate the rings. This puzzle is a masterpiece in design and concept. Created by Vesa Timonen from Finland.
The dark ring has to stand on the silver ring as showed in the picture above, then you can pull piece A away.
Piece A must be rotated so that the thinner end is in the middle of the silver ring, piece B must be rotated as well.
Afterwards piece C can be rotated against the clock at the same time piece C must be pulled to the rigth side. Then you've solved the puzzle.
For the assambly you have to take piece A into your hand as shown on the picture above. Then you take piece C and put it with the thicker end in the middle of the silver ring. Afterwards take piece B and set it with the thicker end to the middle of the silver ring from below. And then you can put all together with a sliding move.