The average cost of some books is $40.
If 9 of the books are now each $15 more and
5 of the books are now each $7 less,
the average cost of the books becomes $44.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	40	44
Total value	40 u	44 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 $15 more and 5 books are now each $7 less
= 9 x 15 - 5 x 7
= 135 - 35
= $100

Total cost of books after 9 books are now each $15 more and 5 books are now each $7 less
= 40 u + 100

Total cost of books in the end
= 44 x 1 u
= 44 u

44 u = 40 u + 100
44 u - 40 u = 100
4 u = 100

1 u = 100 ÷ 4 = 25

Number of books in the end
= 1 u
= 25

Answer(s): 25