There are 20 books in each box. If each box has 20 books, then the number of books is 20 times the number of boxes. (20 books: 1 box, 40 books: 2 boxes...)
So, using mathematical terms, the number of books is equal to the number of boxes x 20.
Here, it says James packs x boxes of books. This means that we call the number of boxes, x. X could be 1,2,3, or any other number.
So, since there are 20 times as many books as boxes, the number of books is 20 times x.
In math terms, this is written like 20x. So the answer is 20x.
