st-1 ==> not sufficient, we definitely the size of thickest book(s). Alternatively, we have smallest size book from which we can determine what space of the shelf is utilized. Now for the remaining space, you can assume like the book is 1 ft thick and very few books will be filled or the book is just 2.1 inches thick... so doesnt help much.
St-2 ==> 22 books require 5.5 ft i.e. 65 inches.
2 books utilized 5 inch each. so remaining 20 books are left with 55 inches.
or approximately 2.75" each book.
we know that 22 books cannot fit in 5 ft but needs 5.5 ft.
so from above we can conclude that (approx.) 2 books are not able to fit and that is why additional 0.5 ft is required.
And in 5 ft shelf, only 20 books can fit.