Since the maximum budget that Jennifer has is $62.10. You can input P=62.10 so now you can start solving.
Steps: 62.10 = (20+0.5x)+0.15(20+0.5x)
62.10 = 20 + 0.5x + 3 + 0.075x
(Combine "like" terms: so for here you combine x together)
62.10 = 0.575x + 20 +3
62.10 = 0.575x + 23
-23 - 23
39.10 = 0.575x
(Now divide 0.575 on both sides.)

You should end up with:
x = 68
Jennifer can have 68 pages in her book to fit her budget.