The average cost of some books is $45.
If 9 of the books are now each $12 more and
4 of the books are now each $4 less,
the average cost of the books becomes $49.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	45	49
Total value	45 u	49 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 4 books are now each $4 less
= 9 x 12 - 4 x 4
= 108 - 16
= $92

Total cost of books after 9 books are now each $12 more and 4 books are now each $4 less
= 45 u + 92

Total cost of books in the end
= 49 x 1 u
= 49 u

49 u = 45 u + 92
49 u - 45 u = 92
4 u = 92

1 u = 92 ÷ 4 = 23

Number of books in the end
= 1 u
= 23

Answer(s): 23