Based on the given table, you can notice that the rate of change of the number of pages read related to the number of hours, is 120 pages each 3 hours.
Then, to determine the number of pages read for 15 hours, consider that after 12 hours Sarah read 480 pages. After 3 more hours, that is, for 15 hours of reading, Sarah read 120 pages more, that is, she read 480+120=600 pages.
Hence, the answer is 600