So in this question, 15 pages * 6 days = 90 pages. Which is 5/8 of the book (since we only have 3/8 left). So we have something like:
(5/8)*x = 90
where x = to the number of pages in the book.
Then (90)/(5/8) = x, which is really like saying 90*(8/5), which is equal to 144.
All in all, there are 144 pages in the book.
