so her album is 30 pages in total, and we know the 3/5 of that are already in use by photos, how much is 3/5 of 30? well, is just their product, namely 18.
since 18 pages are already in use, the leftover is 12 pages, now she wants to set aside 1/4 of those 12 for her vacation, how much is 1/4 of 12? well, is just their product, namely 3.
how many pages are leftover for her photos of her cat and her dog? well 12 - 3 = 9.