The ratio of the number of books Exel had to the number of books Anne had was 4:5.
If Anne had 20 books, then how many books did the both of them have?
well, think of the ratio like a fraction.
4/5 = x/20
we multiply 5 by 4 to get to 20, so we must also multiply the numerator (in this case, 4) by 4, to get the amount of books Exel had.
so, Exel had 16 books, and Anne had 20 books.
hope this helped! sorry if my explanations weren't too clear. if you have any further questions, don't hesitate to ask!