The current no. of books at the book fair is 390. With 5 more boxes, each with 10 books, we will have to add that many number of books to 390.
5 boxes each with 10 books = 5 x 10 = 50
current number of books = 390
total number of books = 390 + 50 = 440
There are 440 books at the book fair.
*I am unsure of what you mean by "count by tens", but if your teacher is asking you how many "ten books" are in the book fair, then there would be 44 sets of 10 books.
