The average cost of some books is $45.
If 4 of the books are now each $15 more and
3 of the books are now each $4 less,
the average cost of the books becomes $48.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	45	48
Total value	45 u	48 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 4 books are now each $15 more and 3 books are now each $4 less
= 4 x 15 - 3 x 4
= 60 - 12
= $48

Total cost of books after 4 books are now each $15 more and 3 books are now each $4 less
= 45 u + 48

Total cost of books in the end
= 48 x 1 u
= 48 u

48 u = 45 u + 48
48 u - 45 u = 48
3 u = 48

1 u = 48 ÷ 3 = 16

Number of books in the end
= 1 u
= 16

Answer(s): 16