Niall has already read 100 pages, and the book has a total of 520 pages. To find out how many pages he has left to read, we can subtract the pages he has read from the total number of pages:
520 pages (total) - 100 pages (read) = 420 pages left.
Now, if Niall plans to read 60 pages per night, we can determine how many nights it will take him to finish the remaining 420 pages by dividing the remaining pages by the number of pages he reads each night:
420 pages รท 60 pages per night = 7 nights.
So, it will take him 7 nights to finish the book.