There are 320 popular science books in the library, accounting for two-fifths of all books, so 2/5*X=320.

When solving the equation, X=800, that is, there are 800 books in the library.

Build a library with y story books.

Popular science books are equivalent to four-thirds of story books, so 4/3*Y=320.

Y=240 when solving the equation, that is, there are 240 story books in the library.

Extended data:

The solution of one-dimensional linear equation;

1, denominator: multiply the least common multiple of each denominator on both sides of the equation.

2. Remove the brackets: first remove the brackets, then remove the brackets, and finally remove the braces: (remember to change the number if there is a minus sign outside the brackets).

3. Move the term: move all the terms containing unknowns to one side of the equation, and all the other terms to the other side of the equation; Move the item to change the symbol.

4. Merge similar terms: transform the equation into the form of ax=b(a≠0).

5. coefficient division 1: divide the unknown coefficient a on both sides of the equation to get the solution of equation x = b/a. ..

Properties of the equation 1: When a number is added to both sides of the equation or the same number or the same algebraic expression is subtracted, the equation is still valid.

Property 2 of the equation: both sides of the equation expand or contract at the same time by the same multiple (except 0), and the equation still holds.

Property 3 of the equation: When both sides of the equation are multiplied (or squared) at the same time, the equation still holds.

Solving the equation is based on these three properties of the equation. One of the properties of the equation: adding a number or subtracting the same number on both sides of the equation at the same time, the equation still holds.