To get access to restricted materials and all problems to solve, pay the tariff.
You choose the problems you want to solve. Keep in mind that there are relatively simple problems (at the beginning of each series) and more complex ones.
In the process of solving the problem, you will be guided by a clear template for solving the problem and you will use hints that explain each step, as well as study theoretical materials that show in detail and with examples how to do this step.
Solving this number of problems will allow you to master the basic skill of solving problem situations.