Out of a total of 9 books arranged in a particular order, the mean of the number of pages in the first 5 books is 430, the mean of the number of pages in the last 5 books is 380 and the mean of the number of pages in all the 9 books is 400. Find the number of pages in the fifth book.

450

The total number of pages in first 5 books = 5 × 430 = 2150

The total number of pages in last 5 books = 5 × 380 = 1900

The total number of pages in all 9 books =  9 × 400 = 3600

So the number of pages in fifth book = 2150 + 1900 - 3600 =  450