Total number of books of the first category:1/(1+2) =1/3.

The number of books in the second category is 1, which is one third of the other three categories.

Total number of books of category II:1/(1+3) =1/4.

The number of books in the third category is 1 of the other three categories.

Total number of books of the third category:1/(1+4) =1/5.

Total number of books in the fourth category:1-1/3-1/4-1/5 =13/60.

The fourth category has 26 books.

These four types of books are: 26÷ 13/60= 120 (volume).

26/(1-1/3-1/4-1/5) =120 (volume) This is correct.