Fernando has 29 books
The prime numbers between 21 and 30 are 23 and 29.
hence, the possible number of books Fernando has 23 or 29.
After taking 4 books away, the total number of books left can be :-
Since, 19 is not a square number so he cannot be able to place the same number of books on each shelf of his bookcase.
So Fernado doesn't have 23 books.
Here, 25 is a square number so he will be able to place the same number of books on each shelf of his bookcase.
Therefore, Fernando has 29 books.
23 and 29
From googl- I mean me
29 books ; 5 bookshelfs
Solving logically ;
Prime numbers between 21 and 30 = 23, 29
Hence, number of books Fernando has is either 23 or 29
Taking 4 away from the number of books, he'll be able to arrange same number of books on each of his book shelve;
From the statement, we can conclude that :
Subtracting 4 from the number of books Fernando has should give a perfect square
If 23 books
23 - 4 = 19 ; 19 isn't a perfect square
If 29 books :
29 - 4 = 25
Hence, to place the same Number of books on each shelf :
Sqrt(25) = 5
Hence, number of bookshelf = 5
The answer would be 26 because the only prime numbers on from 21 to 30 are 23 and 29 ans 29 - 4 equals 26 and 23 - 4 is 19