The average cost of some books is $30.
If 6 of the books are now each $12 more and
3 of the books are now each $6 less,
the average cost of the books becomes $33.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	30	33
Total value	30 u	33 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 6 books are now each $12 more and 3 books are now each $6 less
= 6 x 12 - 3 x 6
= 72 - 18
= $54

Total cost of books after 6 books are now each $12 more and 3 books are now each $6 less
= 30 u + 54

Total cost of books in the end
= 33 x 1 u
= 33 u

33 u = 30 u + 54
33 u - 30 u = 54
3 u = 54

1 u = 54 ÷ 3 = 18

Number of books in the end
= 1 u
= 18

Answer(s): 18