It is level 81, so you have to touch the numbers generate value 81.
So you are not confused, first turn your phone 180 degrees, then tap numbers 9x9 (9 x 9 = 81) in the box wherever you like, and then press the "=" the door will open.
We proceed with the explanation the level of 82. Open a hole in the floor right by using a knife, then grab the dragon's eyes and keep them in your inventory, then keep an eye on the dragon circle left of the door.
Turn left eye dragon showed up at 9, then turn the dragon's eye right up to show time 4. A panel should open with a lever in it.
Take lever of the panel and save it, then move the lever to the right side wall of the door, press the lever to open the door level 82
Now continue to the 100 floors level 83-84 explanation, or back to the 100 floors level 80 explanation