The average number of pages of nine book is 400. If the average number of pages of the first five books is 430 and that of last five books is 380, then what is the number of pages of the fifth book?

450

Sum of pages of 9 books = 9 × 400 = 3600

Sum of pages of first 5 books = 5 × 430 = 2150

Sum of pages of last 5 books = 5 × 380 = 1900

Pages in 5th book = ( 2150 + 1900 ) - 3600 = 450