The average cost of some books is $60.
If 9 of the books are now each $8 more and
2 of the books are now each $5 less,
the average cost of the books becomes $62.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	60	62
Total value	60 u	62 u

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

Total cost of the books at first
= 60 x 1 u
= 60 u

Increase in the cost after 9 books are now each $8 more and 2 books are now each $5 less
= 9 x 8 - 2 x 5
= 72 - 10
= $62

Total cost of books after 9 books are now each $8 more and 2 books are now each $5 less
= 60 u + 62

Total cost of books in the end
= 62 x 1 u
= 62 u

62 u = 60 u + 62
62 u - 60 u = 62
2 u = 62

1 u = 62 ÷ 2 = 31

Number of books in the end
= 1 u
= 31

Answer(s): 31