The average cost of some books is $25.
If 5 of the books are now each $14 more and
4 of the books are now each $8 less,
the average cost of the books becomes $27.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	25	27
Total value	25 u	27 u

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

Total cost of the books at first
= 25 x 1 u
= 25 u

Increase in the cost after 5 books are now each $14 more and 4 books are now each $8 less
= 5 x 14 - 4 x 8
= 70 - 32
= $38

Total cost of books after 5 books are now each $14 more and 4 books are now each $8 less
= 25 u + 38

Total cost of books in the end
= 27 x 1 u
= 27 u

27 u = 25 u + 38
27 u - 25 u = 38
2 u = 38

1 u = 38 ÷ 2 = 19

Number of books in the end
= 1 u
= 19

Answer(s): 19