We do not know which text-book you use, and not sure if we have the book even if we knew.
If you have difficulties finding a problem similar to the cited examples, post what you have, as well as the examples. We can better help you that way.
I am a tutor for a student working on the same thing so I somewhat know this material. (the book is Introductory and Intermediate Algebra for College Students, by the way)
Unfortunately I don't have the book, so I am using my general knowledge and the girl I tutor sent me a screen shot of one page. The unfortunate thing is that I still cannot help her share a problem with the class because we can't flip through the book.
How does the author determine what the first equation should be?
The first step is to make a total value problem. This step helps us familiarize ourselves with the information given and makes it easier to understand.
What about the second equation?
The second equation is where they put in more general information from the word problem and make it a system of equations.
How are these examples similar?
They are similar because they both take the word problems and change the information into equations. They are similar because they use the same letters and they are trying to solve the values of each variable.
How are they different?
They are different because they use completely different facts from the information.