Solution: Suppose there are x fairy tales and 4X story books.

So in the sense of the problem, X+4X=90,

X= 18，

So there are 18 fairy tales, and 18×4=72 story books.

Method 2

Solution: Because storybooks are four times as big as fairy tales, the ratio between them is 4: 1.

So the story book is: 90×4/( 1+4)=90×4/5=72 (this),

The book of fairy tales is: 90×1(1+4) = 90×1/5 =18 (Ben).