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.
Solution
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
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