The average cost of some books is $50.
If 9 of the books are now each $13 more and
3 of the books are now each $4 less,
the average cost of the books becomes $55.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	50	55
Total value	50 u	55 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 9 books are now each $13 more and 3 books are now each $4 less
= 9 x 13 - 3 x 4
= 117 - 12
= $105

Total cost of books after 9 books are now each $13 more and 3 books are now each $4 less
= 50 u + 105

Total cost of books in the end
= 55 x 1 u
= 55 u

55 u = 50 u + 105
55 u - 50 u = 105
5 u = 105

1 u = 105 ÷ 5 = 21

Number of books in the end
= 1 u
= 21

Answer(s): 21