The average cost of some books is $25.
If 9 of the books are now each $11 more and
5 of the books are now each $6 less,
the average cost of the books becomes $28.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	25	28
Total value	25 u	28 u

Number of books at first = 1 u
Number of books in the end = 1 u

Total cost of the books at first
= 25 x 1 u
= 25 u

Increase in the cost after 9 books are now each $11 more and 5 books are now each $6 less
= 9 x 11 - 5 x 6
= 99 - 30
= $69

Total cost of books after 9 books are now each $11 more and 5 books are now each $6 less
= 25 u + 69

Total cost of books in the end
= 28 x 1 u
= 28 u

28 u = 25 u + 69
28 u - 25 u = 69
3 u = 69

1 u = 69 ÷ 3 = 23

Number of books in the end
= 1 u
= 23

Answer(s): 23