The average cost of some books is $40.
If 5 of the books are now each $15 more and
3 of the books are now each $5 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 5 books are now each $15 more and 3 books are now each $5 less
= 5 x 15 - 3 x 5
= 75 - 15
= $60

Total cost of books after 5 books are now each $15 more and 3 books are now each $5 less
= 40 u + 60

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

44 u = 40 u + 60
44 u - 40 u = 60
4 u = 60

1 u = 60 ÷ 4 = 15

Number of books in the end
= 1 u
= 15

Answer(s): 15