Step: 1
The part of the book read by William on the first day = 17
Step: 2
The part of book read by him on the second day = 47
Step: 3
The total part of the book read by him is 17 + 47.
Step: 4
The denominators are same. So, add the numerators.
Step: 5
Step: 6
William has finished reading 57th part of the book.