Hello everybody!

I have this question. KAPLAN says that there will be 3 to 5 passages on the reading section on TOEFL. Then it gives such breakdown:
1st passage - 20 minutes.
2nd and 3rd - 40 minutes.
4th and 4th - 40 minutes.
I wonder why the timelines are given. As far as know I can easily surf forward and back through questions and passages, right? Do these timeslots mean that when I'm done with the first passage all the questions from it become unavailable and I can't amend them?
The reason I'm asking is because according to Princeton "Cracking the..." there are 3 questions' difficulty levels, so I'm going to go through the easiest first, then move on to the next level of difficulty. And I'm here trying to figure out whether I should go through the easiest ones for ALL passages, or answer the questions of all levels of diffictulties for each passage given.
Thanks in advance!