The average cost of some books is $30.
If 7 of the books are now each $15 more and
6 of the books are now each $5 less,
the average cost of the books becomes $33.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	30	33
Total value	30 u	33 u

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

Total cost of the books at first
= 30 x 1 u
= 30 u

Increase in the cost after 7 books are now each $15 more and 6 books are now each $5 less
= 7 x 15 - 6 x 5
= 105 - 30
= $75

Total cost of books after 7 books are now each $15 more and 6 books are now each $5 less
= 30 u + 75

Total cost of books in the end
= 33 x 1 u
= 33 u

33 u = 30 u + 75
33 u - 30 u = 75
3 u = 75

1 u = 75 ÷ 3 = 25

Number of books in the end
= 1 u
= 25

Answer(s): 25