Then the first category/total number of books = 1/3 (line chart can help to understand)
The second category is 1/3 of the other three categories.
Category II/Total number of books = 1/4.
The third category is 1/4 of the other three categories.
Category III/Total number of books = 1/5.
So the fourth category =1-1/3-1/4-1/5 =13/60.
There are 26 known books in the fourth category.
Total number of books =26÷ 13/60= 120 (Ben)
A: These four books have a total of 120 volumes.
-Answer.