The average cost of some books is $35.
If 8 of the books are now each $11 more and
2 of the books are now each $8 less,
the average cost of the books becomes $38.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	35	38
Total value	35 u	38 u

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

Total cost of the books at first
= 35 x 1 u
= 35 u

Increase in the cost after 8 books are now each $11 more and 2 books are now each $8 less
= 8 x 11 - 2 x 8
= 88 - 16
= $72

Total cost of books after 8 books are now each $11 more and 2 books are now each $8 less
= 35 u + 72

Total cost of books in the end
= 38 x 1 u
= 38 u

38 u = 35 u + 72
38 u - 35 u = 72
3 u = 72

1 u = 72 ÷ 3 = 24

Number of books in the end
= 1 u
= 24

Answer(s): 24