The average cost of some books is $60.
If 9 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 $65.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	60	65
Total value	60 u	65 u

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

Total cost of the books at first
= 60 x 1 u
= 60 u

Increase in the cost after 9 books are now each $15 more and 3 books are now each $5 less
= 9 x 15 - 3 x 5
= 135 - 15
= $120

Total cost of books after 9 books are now each $15 more and 3 books are now each $5 less
= 60 u + 120

Total cost of books in the end
= 65 x 1 u
= 65 u

65 u = 60 u + 120
65 u - 60 u = 120
5 u = 120

1 u = 120 ÷ 5 = 24

Number of books in the end
= 1 u
= 24

Answer(s): 24