To determine the total number of books Lena has , we must analyse the mathematical constraints provided by the exact average number of pages and the condition in the statements.
Leena's avg number of pages per book is exactly 252.08. This can be written as a mixed fraction
252+8/100 = 252+2/25 = 6302/25
Because the total number of books (n) and total number of pages (T) must both be the integers
Avg = T/n = 6302/25
For the total pages (T) to be a whole number the number of books (n) must be a multiple of 25.
ST 1 : each Shelf contains between 2 and 7 books , inclusive .
Lena has 7 shelves
Min Books : 7 shelves x 2 books / shelf = 14 books .
Max books : 7 Shelves x 7 books /shelf = 49 books.
Within the possible range of 14 to 49 books , the only multiple of 25 is 25.
This definitely identifies that lena has exactly 25 books. Sufficient ✅
ST 2: The total number of pages of her books is less than 10,000
If n = 25 , Total Pages = 25 x 252.08 = 6302 ( This is less than 10,000)
If n = 50 , Total pages = 50 x 252.08 = 12604 ( This is greater than 10,000)
Any higher multiple of 25 will also exceed 10,000 pages.
This confirms that lena must have exactly 25 books.
Sufficient.✅