Can you recommend a mathematical logic textbook that is easy to understand?

I started studying "Introduction to Mathematical Logic, Sixth Edition" by Elliot Mendelson. I tried to not read solutions as long as possible, but most exercises were awfully difficult to me, and the content itself was not easily digestible.

When I tried to solve exercises in the chapter about many valued logic, I couldn't even think of any approach of an exercise for half an hour, so I became impatient and read the solution. Some of the solutions were even cryptic as hell as below.

What? There are n^n possible truth functions of one variable? The book never taught me how to calculate the number of possible truth functions of one variable or more. I was confused a lot.

I study math alone at home, and I feel that I am probably not ready for this book if I find myself resorting to solutions often, and the solutions are difficult to understand.

The author claims "I believe that the essential parts of the book can be read with ease by anyone with some experience in abstract mathematical thinking. There is, however, no specific prerequisite.", and I had some experience in abstract mathematical thinking from "how to prove it, 2nd edition" by velleman. But, I feel that it is not really an introduction for someone who learns logic for the first time.

Can you recommend some other textbooks that can replace the book and that matches my level of math?

Have a look at A first course in mathematical logic and set theory by Michael O'Leary. It does not cover too much too fast.