The average cost of some books is $30.
If 9 of the books are now each $12 more and
7 of the books are now each $6 less,
the average cost of the books becomes $32.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	30	32
Total value	30 u	32 u

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

Total cost of the books at first
= 30 x 1 u
= 30 u

Increase in the cost after 9 books are now each $12 more and 7 books are now each $6 less
= 9 x 12 - 7 x 6
= 108 - 42
= $66

Total cost of books after 9 books are now each $12 more and 7 books are now each $6 less
= 30 u + 66

Total cost of books in the end
= 32 x 1 u
= 32 u

32 u = 30 u + 66
32 u - 30 u = 66
2 u = 66

1 u = 66 ÷ 2 = 33

Number of books in the end
= 1 u
= 33

Answer(s): 33