The average cost of some books is $50.
If 8 of the books are now each $13 more and
4 of the books are now each $6 less,
the average cost of the books becomes $52.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	50	52
Total value	50 u	52 u

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

Total cost of the books at first
= 50 x 1 u
= 50 u

Increase in the cost after 8 books are now each $13 more and 4 books are now each $6 less
= 8 x 13 - 4 x 6
= 104 - 24
= $80

Total cost of books after 8 books are now each $13 more and 4 books are now each $6 less
= 50 u + 80

Total cost of books in the end
= 52 x 1 u
= 52 u

52 u = 50 u + 80
52 u - 50 u = 80
2 u = 80

1 u = 80 ÷ 2 = 40

Number of books in the end
= 1 u
= 40

Answer(s): 40