How to pick start and end?

#1

i dont understand , in some question we start from zero and in some questions we start from arr[0] and end=arr[n-1]-arr[0].

can you plz explain, with example ? when to choose which ?

#2

For binary search way can start from 0 to sum_of_all_pages as the answer lies between this range. While validating a particular value of pages we start from arr[0] and check whether such distribution is possible.