When approaching one of these doors and revealing the information on it, you will notice that there are numbers and pictures. Around the outside of the door, there are ten symbols which start from the bottom left at (0) to the bottom right at (9).
To solve these doors, you must first add up the number on the outside with the other number or picture value to get in the triangle to get your today. As seen in the video, you must find the value of the icon (2) and plus it to the number (11) which makes (13) and then subtract that number from the number in the middle (21) which means that 21 - 13 = 8. This will mean that the first '?' icon will be the 8th picture that is around the wall.
If you get doors where there are two icons rather than a number, you will simply add the values of these icons together and then subtract them from the middle number as before.