The average cost of some books is $60.
If 7 of the books are now each $10 more and
4 of the books are now each $4 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 7 books are now each $10 more and 4 books are now each $4 less
= 7 x 10 - 4 x 4
= 70 - 16
= $54

Total cost of books after 7 books are now each $10 more and 4 books are now each $4 less
= 60 u + 54

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

62 u = 60 u + 54
62 u - 60 u = 54
2 u = 54

1 u = 54 ÷ 2 = 27

Number of books in the end
= 1 u
= 27

Answer(s): 27