The ratio of 8 books to 20 books is
2 : 5
5 : 2
4 : 5
5 : 4
So, the ratio of 8 books to 20 books in simplest form is (2 : 5).
(5:2) means ( dfrac{5}{2} ), which is larger than (1); but ( dfrac{8}{20} = 0.4 ) is less than (1).
(4:5) means ( dfrac{4}{5} = 0.8 ), not (0.4).
(5:4) means ( dfrac{5}{4} = 1.25 ), not (0.4).
Therefore, only (2:5) matches.