Due to some unavoidable reason I cannot go to university and I don't have access to good teachers where I live. I have completed my high school with good grades and determined to self-study undergraduate mathematics including calculus, combinatorics, number theory, graph theory, probability, etc.
The problem I face while studying from books is whenever I cannot solve a particular question, I have no access to step by step solutions.
Why do most mathematics books don't contain solutions? How can I deal with this situation?
When you were in your undergrad years, How did you cope with lack of solutions in textbooks? Did your professors help you?
And as most books have huge number of questions asking them all here on StackExchange does not seem feasible. How to deal with this situation??
I think I can speak to this. First, for the traditional calculus sequence in the U.S. most step-by-step solutions can be found either on chegg study (monthly subsciription cost) or slater. Additionally, for a first course in differential equations and linear algebra, the same can be found. Most mathematics textbooks contain solutions to odd-numbered problems but not step-by-step solutions, that's when these other resources can be useful. Considering you don't have a professor to show you the way, I would definitely recommend these resources, as they greatly helped me. Sometimes, they even offer a different way of doing a problem that is easier to follow. However, I'd definitely recommend trying your hardest to figure out how to solve the problem before looking at step-by-step solutions, as it will pay off in the long run. Good luck!