The average cost of some books is $60.
If 9 of the books are now each $9 more and
2 of the books are now each $6 less,
the average cost of the books becomes $63.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	60	63
Total value	60 u	63 u

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

Total cost of the books at first
= 60 x 1 u
= 60 u

Increase in the cost after 9 books are now each $9 more and 2 books are now each $6 less
= 9 x 9 - 2 x 6
= 81 - 12
= $69

Total cost of books after 9 books are now each $9 more and 2 books are now each $6 less
= 60 u + 69

Total cost of books in the end
= 63 x 1 u
= 63 u

63 u = 60 u + 69
63 u - 60 u = 69
3 u = 69

1 u = 69 ÷ 3 = 23

Number of books in the end
= 1 u
= 23

Answer(s): 23