3 x 4 = 12
(1/3) x 12u = 4u ——- 5 books + 11 files
1 book ——- 3 files
5 books ——- 15 files
4u ——- 15 + 11 = 26 files
1u ——- 6.5 files
12u – 4u = 8u
(3/4) x 8u = 6u ——- 6.5 x 6 = 39 files
39/3 = 13 books
13 + 5 = 18

Ans : 18 books.