The average cost of some books is $35.
If 8 of the books are now each $15 more and
6 of the books are now each $7 less,
the average cost of the books becomes $37.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	35	37
Total value	35 u	37 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 $15 more and 6 books are now each $7 less
= 8 x 15 - 6 x 7
= 120 - 42
= $78

Total cost of books after 8 books are now each $15 more and 6 books are now each $7 less
= 35 u + 78

Total cost of books in the end
= 37 x 1 u
= 37 u

37 u = 35 u + 78
37 u - 35 u = 78
2 u = 78

1 u = 78 ÷ 2 = 39

Number of books in the end
= 1 u
= 39

Answer(s): 39