The solution process is as follows: there are 45 books in the known corner, two fifths of which are story D books, that is, story books account for two fifths of the total Z. To find the number of story books, multiplication is needed, and the mathematical expression is: 45×2/5.

45×2/5= 18 (Ben)

Extended data:

General situation of steps to solve the problem of score application

1, find out whose unit "1" is. First of all, we must understand the meaning of the score. Generally speaking, there are two key words: before watching and after watching. Look for these two keywords in the narrative of the application problem.

2. If "1" company knows how to use multiplication, then "1" company doesn't know how to use division. In other words, the quantity of the unit "1" is calculated by division instead of multiplication.

3. The amount of judgment must correspond to the rate. Fraction application problems can be summarized into three types:

Ordinary type, increasing type and decreasing type. Quantity and rate must correspond, the increasing rate is (1+ increasing rate) and the decreasing rate is (1- decreasing rate).