Suppose Arif can read 39 pages of a book in half an hour. How long will It take her to finish a book that is 1287 pages?

Arif can Read 39 pages in 30 minutes.
How much time is required to read 1287 pages?
39 page read time = 30 minute
01 page read time=30/39=0.76923 minute
1287 pages read time=0.769*1287=990 minute
990 conversion into hours = 990/60 = 16.5 Hours

Another Simple Explanation:

39 pages for a half hour equals 78 pages per hour and if the book has 1287 pages then 1287 pages divided by 78 pages per hour gives you 16.5.