The average cost of some books is $45.
If 8 of the books are now each $13 more and
3 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 8 books are now each $13 more and 3 books are now each $4 less
= 8 x 13 - 3 x 4
= 104 - 12
= $92

Total cost of books after 8 books are now each $13 more and 3 books are now each $4 less
= 45 u + 92

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

47 u = 45 u + 92
47 u - 45 u = 92
2 u = 92

1 u = 92 ÷ 2 = 46

Number of books in the end
= 1 u
= 46

Answer(s): 46