About the level system
All the material in our course is grouped into levels, from the simplest to the most complex. The levels are numbered in consecutive numbers (1, 2, 3, ...), sublevels numbered with letters are also introduced within the level (in level 1 — sublevels 1A, 1B, 1C and 1D, etc.)
Most levels have the following structure: sublevels A and B contain new material, and sublevels C and D (the latter is not always there) — additional problems of increased complexity to consolidate this material. Sometimes in levels C and D there may be particularly difficult problems for the material of significantly earlier levels (for example, in level 5C there may be a difficult problem for the material of level 3). Sometimes there are contests on separate topics in levels C and D, sometimes additional problems in these levels go in random order.
I assume that you will go through the sublevels sequentially, moving to a new sublevel usually when you have "passed" the previous sublevel. Exception: if you have passed some topic in sublevel A or B, and you see a contest of advanced problems on the same topic in sublevel C or D, then you can solve it.
The requirements for the "passing" of each sublevel are indicated under its heading. As a rule, for sublevels A and B it is required to solve all the problems, for levels C and D it is required to solve part (half or a third) of the problems. However, this does not mean that the remaining problems of levels C and D do not need to be solved. I recommend that if you have solved the necessary minimum of problems of levels C and D, go to the next level in order to quickly begin studying a new theory. But at the same time, return regularly to the unfinished problems of the previous levels. It can be estimated approximately as follows: when you move to level 4A, you should have solved half of the problems of level 3C, 3/4 of the problems of level 2C and about 7/8 of the problems of levels 1C and 1D. Similarly, when you move to level 5A, you should have solved half of the problems of level 4C, as well as 3/4 of the problems of level 3C, 7/8 of the problems of level 2C and almost all (with the rare exception of particularly unpleasant problems) problems of levels 1C and 1D. Similarly for other levels.
Please note that in levels C and D, the problems are not always ordered by complexity. Therefore, solve them in the order in which it is more convenient for you!