Answer:
9 pages she can fill before her vacation.
Step-by-step explanation:
Total pages in album = 30
Fraction of photo album pages which are is full =
Photo album pages which are is full =
Fraction of photo album pages which are empty :x
(Sum of all fraction is 1)

Album pages which are empty =
Fraction of album set aside for vacation photos =
of the photo album pages which are empty
Album pages which are kept for vacation photos:
=
Total empty pages left = 30 - 18 - 3 = 9
9 pages she can fill before her vacation.