The average cost of some books is $40.
If 9 of the books are now each $13 more and
3 of the books are now each $6 less,
the average cost of the books becomes $43.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	40	43
Total value	40 u	43 u

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

Total cost of the books at first
= 40 x 1 u
= 40 u

Increase in the cost after 9 books are now each $13 more and 3 books are now each $6 less
= 9 x 13 - 3 x 6
= 117 - 18
= $99

Total cost of books after 9 books are now each $13 more and 3 books are now each $6 less
= 40 u + 99

Total cost of books in the end
= 43 x 1 u
= 43 u

43 u = 40 u + 99
43 u - 40 u = 99
3 u = 99

1 u = 99 ÷ 3 = 33

Number of books in the end
= 1 u
= 33

Answer(s): 33