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.

the mean of the number of pages in the first 5 books is 430.

total number of pages in the first 5 books = $$430\times5$$

= 2150    Eq.(i)

the mean of the number of pages in the last 5 books is 380.

total number of pages in the last 5 books = $$380\times5$$

= 1900    Eq.(ii)

the mean of the number of pages in all 9 books is 400.

total number of pages in all 9 books = $$400\times9$$

= 3600    Eq.(iii)

Number of pages in the fifth book = Eq.(i)+Eq.(ii)-Eq.(iii)

= 2150+1900-3600

= 4050-3600

= 450