The average cost of some books is $45.
If 9 of the books are now each $8 more and
5 of the books are now each $4 less,
the average cost of the books becomes $47.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	45	47
Total value	45 u	47 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 $8 more and 5 books are now each $4 less
= 9 x 8 - 5 x 4
= 72 - 20
= $52

Total cost of books after 9 books are now each $8 more and 5 books are now each $4 less
= 45 u + 52

Total cost of books in the end
= 47 x 1 u
= 47 u

47 u = 45 u + 52
47 u - 45 u = 52
2 u = 52

1 u = 52 ÷ 2 = 26

Number of books in the end
= 1 u
= 26

Answer(s): 26