The average cost of some books is $55.
If 9 of the books are now each $10 more and
3 of the books are now each $8 less,
the average cost of the books becomes $58.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	55	58
Total value	55 u	58 u

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

Total cost of the books at first
= 55 x 1 u
= 55 u

Increase in the cost after 9 books are now each $10 more and 3 books are now each $8 less
= 9 x 10 - 3 x 8
= 90 - 24
= $66

Total cost of books after 9 books are now each $10 more and 3 books are now each $8 less
= 55 u + 66

Total cost of books in the end
= 58 x 1 u
= 58 u

58 u = 55 u + 66
58 u - 55 u = 66
3 u = 66

1 u = 66 ÷ 3 = 22

Number of books in the end
= 1 u
= 22

Answer(s): 22