To find the number of days it will take the class to finish the book, we can divide the total number of pages by the number of pages read per day:
Number of days = Total number of pages / Number of pages read per day
Plugging in the given values, we get:
Number of days = 318 / 15 ≈ 21.2
Since we cannot have a fraction of a day, we round up to the nearest whole number of days. Therefore, it will take the class about 22 days to finish the book.