Niall has already read 100 pages, so the number of pages left to read is:
520 - 100 = 420 pages.
If he plans to read 60 pages per night, the number of nights it will take him to finish the book can be calculated as follows:
420 pages รท 60 pages/night = 7 nights.
Therefore, it will take him 7 nights to finish the book.