The average cost of some books is $45.
If 9 of the books are now each $12 more and
6 of the books are now each $5 less,
the average cost of the books becomes $48.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	45	48
Total value	45 u	48 u

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

Total cost of the books at first
= 45 x 1 u
= 45 u

Increase in the cost after 9 books are now each $12 more and 6 books are now each $5 less
= 9 x 12 - 6 x 5
= 108 - 30
= $78

Total cost of books after 9 books are now each $12 more and 6 books are now each $5 less
= 45 u + 78

Total cost of books in the end
= 48 x 1 u
= 48 u

48 u = 45 u + 78
48 u - 45 u = 78
3 u = 78

1 u = 78 ÷ 3 = 26

Number of books in the end
= 1 u
= 26

Answer(s): 26