Let's assume the book (not the book cover) is 480 pages long. After 1.5 hours, she had read (480-402) pages: 78 pages in 1.5 hours.
Thus, her reading rate is 78 pages / 1.5 hours, or 52 pages / hr.
Then P = the # of pages left to read = 480 pages - (52 pages/hr)*t