Let "x" be the number of students in the class, and "b" be the number of books donated to the class.

We are given two conditions:

If each student takes 4 books, there will be 20 books left. This can be expressed as: x * 4 + 20 = b

If 3 students do not take a book and the rest take 5 books each, there will be no books left. This can be expressed as: (x - 3) * 5 = b

Now, we have a system of two equations:

1. x * 4 + 20 = b
2. (x - 3) * 5 = b

We can solve this system of equations to find the values of "x" and "b."

First, let's simplify equation 2:

5x - 15 = b

Now, we have the system:

1. x * 4 + 20 = b
2. 5x - 15 = b

Since both equations equal "b," we can set them equal to each other:

x * 4 + 20 = 5x - 15

Now, solve for "x":

4x + 20 = 5x - 15

Subtract 4x from both sides:

20 = x - 15

Add 15 to both sides:

x = 35

So, there are 35 students in the class. Now, we can find the number of books donated (b) using either of the original equations. Let's use the first one:

x * 4 + 20 = b 35 * 4 + 20 = b 140 + 20 = b b = 160

Therefore, 160 books were donated to the class.