The average cost of some books is $50.
If 6 of the books are now each $14 more and
4 of the books are now each $5 less,
the average cost of the books becomes $54.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	50	54
Total value	50 u	54 u

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

Total cost of the books at first
= 50 x 1 u
= 50 u

Increase in the cost after 6 books are now each $14 more and 4 books are now each $5 less
= 6 x 14 - 4 x 5
= 84 - 20
= $64

Total cost of books after 6 books are now each $14 more and 4 books are now each $5 less
= 50 u + 64

Total cost of books in the end
= 54 x 1 u
= 54 u

54 u = 50 u + 64
54 u - 50 u = 64
4 u = 64

1 u = 64 ÷ 4 = 16

Number of books in the end
= 1 u
= 16

Answer(s): 16