If we represents a book's cost as x, that would mean that 5*x would represent the cost of 5 books (as x+x+x+x+x=5x) and 4x would be 4, resulting in 5x+4x=9x. After that, we get that 19 books is 19x as the cost, but since Agnus gets a 10 dollar discount, it's 19x-10. After that, to combine the two, we notice that Agnus still spends 5$ more than what they spent combined (9x) so we get that 9x+5=19x-10. Subtracting 9x from both sides, we get 10x-10=5. Adding 10 to both sides there, we get 10x=15 and we can divide 10 by both sides to get x=1.5